Assailants threw a green antiseptic solution on Ukrainian opposition leader Arseniy Yatsenyuk and his deputy, Oleksandr Turchynov, after they visited former Prime Minister Yulia Tymoshenko in a hospital in Kharkiv on February 12.
One of the attackers escaped and another was apprehended by police.
Yatsenyuk, the chairman of Tymoshenko's Batkivshchyna (Fatherland) faction, had held talks with Tymoshenko, who has been jailed on abuse-of-office charges widely seen as politically motivated.
Tymoshenko is undergoing treatment for back pain.
Meanwhile, Russian Foreign Minister Sergei Lavrov warned in an article in the Russian daily "Kommersant" on February 13 that outside attempts to influence Ukraine's future will end "in failure."
Protesters have been demonstrating against President Viktor Yanukovych's government since November, when he scuppered a key deal with the EU in favor of closer ties with Russia.
Based on reporting by Interfax and "Kyiv Post"