The Moldovan authorities have strongly condemned the vote. While the international community has largely shunned the poll, Moscow has not said whether it will recognize the results of the referendum.
Similar polls were held in 1990 and 1991 to create the self-styled Dniester Republic and declare independence from Moldova.
Pro-Russian secessionists fought a short but bloody war with Moldova in the summer of 1992. The fighting left some 1,000 people dead and was halted by Russian troops stationed in Transdniester.
Since then, no country has recognized Transdniester.
Negotiations on Transdniester's final status have been on and off for more than a decade under mediation from Russia, Ukraine, and the Organization for Security and Cooperation in Europe (OSCE). More recently, the United States and the European Union have also gained observer status at the talks.
The region receives unofficial but strong support from Russia, and two-thirds of its 555,000 citizens speak either Russian or Ukrainian. Many hold Russian passports. Some 1,500 Russian troops are still deployed in Transdniester, despite Russia's pledge to withdraw all its forces and equipment by 2002.
The referendum consists of two mutually exclusive questions: "Do you support the course toward the Dniester Republic's independence and ensuing future free accession to the Russian Federation?" And "Do you consider it possible that the Dniester Republic give up its independence and then join Moldova?
Independence and eventual unification with Russia are expected to win overwhelming backing in Sunday's vote. The only problem could be the turnout, since many of Transdniester's almost 400,000 eligible voters are working abroad.
In order to be valid, a simple majority of "yes" votes is needed, provided the turnout is over 50 percent. Some 262 polling stations have been prepared and early voting has been allowed five days in advance.
Moldova has adopted a parliamentary statement condemning the vote. And the international community has declared the referendum illegal, and called on the separatist leadership to resume negotiations, which have been suspended for half a year.
The head of the OSCE mission to Moldova, Louis O'Neill, told RFE/RL that the poll is illegitimate.
"The OSCE will not recognize this referendum, and we have no intention to support or observe a unilateral action, which calls into question the territorial integrity and sovereignty of the Republic of Moldova," O'Neill said. "Particularly when you consider the suggestive character of the questions, which are themselves compound questions, each one of them contains two parts, so there really should be four questions, and that they pretty much imply the desired answer."
The European Union and the United States have also shunned the vote. Emma Udwin, spokeswoman to the EU external relations commissioner Benita Ferrero-Waldner, said the referendum will not change the status quo.
"We don't recognize Transdniester as a state, we don't recognize Transdniester's independence and there is no country that does," Udwin said. "This referendum, which will be held doesn't alter any part of that state of affairs. It will not be recognized by the EU, we understand that it will not be recognized by the OSCE, and therefore it is not something that will have international validity."
David J. Kramer, deputy assistant secretary of state for European and Eurasian Affairs, said no one should question whether Transdniester is part of Moldova.
So far, Russia has not said whether it will recognize the results of the poll. The Russian Foreign Ministry today issued a statement saying that referendums were "seen in recognized democratic states as an important legal basis for building civil society."
However, some Russian officials have spoken in favor of recognizing the Transdniester referendum and an upcoming similar poll. On November 12, Georgia's separatist pro-Moscow region of South Ossetia is holding a referendum.
Konstantin Zatulin, the director of the Institute of CIS Countries and a Duma deputy from the pro-Kremlin Unified Russia party, said Russia is generally in favor of referendums.
"Russia definitely respects the principle of referendums to decide the fate of nations and populations. In my opinion, Russia is totally prepared to recognize, under certain conditions, the independence of Transdniester from the Moldovan republic, especially since this independence has long been a fact," Zatulin said.
Abkhazia, another Moscow-backed separatist region, has also re-affirmed its independence from Georgia. The three pro-Russian territories cite the precedent of Montenegro, which broke off from its union with Serbia through a referendum in June.
The EU's Emma Udwin does not think the analogy is a good one: "We do not see a parallel between the Transdniestrian referendum and the Montenegrin referendum for the very simple reason that there was a contractual agreement between Serbia and Montenegro that such a referendum could take place and would be recognized -- that is not the case with Transdniester."
Officials in the separatist regions are also closely following talks on the future status of Kosovo.
Click on the map to see how many Russians live in each of the former Soviet republics.
RUSSIANS OUTSIDE OF RUSSIA: A total of some 30 million ethnic Russians remain in the republics of the former Soviet Union, including large diasporas in Ukraine, Kazakhstan, and Belarus. This historical legacy has often been a source of tension between Russia and its neighbors. "Support for the rights of compatriots abroad is a crucial goal," Russian President Vladimir Putin said in his April 2005 state-of-the-nation address. "It cannot be subject to a diplomatic or political bargaining. Those who do not respect, observe, or ensure human rights have no right to demand that human rights be respected by others."
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