Speaking after a meeting with Rakhmonov in the Tajik capital, Dushanbe, De Gucht said "we also proposed that we would work together in establishing standards for those elections."
He described the elections as "a very important benchmark on Tajikistan's road to democracy," RFE/RL's Tajik Service reports.
De Gucht arrived in Tajikistan from Turkmenistan, where he had talks with Turkmen President Saparmurat Niyazov on March 29.
The Belgian foreign minister visited Kyrgyzstan on March 28, and Kazakhstan on March 27. He is expected to head for Afghanistan later this week.
The OSCE says it is the world's largest regional security organization, with 55 members in Europe, Central Asia, and North America.