Rebels in the Ukrainian east and the central authorities in Kiev announced plans to exchange prisoners on Sunday.
"The procedure will move along the 30 for 30 formula. We expect the exchange to take place around 5 p.m. Moscow time," a source in the Donetsk people's republic told Interfax.
On Sunday, Ukraine's President Petro Poroshenko tweeted that 12 Ukrainian military servicemen had been released the night before.
The swap will be conducted under the agreement from Sept. 5 stipulating that "all hostages and unlawfully detained prisoners" must be released by both sides.
Confirmation that the exchange had occurred had not yet been reported by the time of publication.