Officials have been planning to ditch the station in a controlled descent that will send it hurtling into a remote area of the Pacific Ocean, 1,500-2,000km east of Australia.
Mission control lost contact with Mir for some 20 hours from 1840 (1540 GMT) on Monday, and after a brief contact around lunchtime on Tuesday, lost it again.
The accident-prone station has never been out of contact for so long before; regular radio contact is essential to control its position.
'Downfall unlikely'
Oofficials played down suggestions that Mir could fall to earth. "The situation isn't catastrophic yet," said spokeswoman Irina Manshilina.
Officials have been planning to ditch the station in a controlled descent that will send it hurtling into a remote area of the Pacific Ocean, 1,500-2,000km east of Australia.
There have been fears that because of its age and unreliability, that descent could go awry.
But as recently as Monday, flight deputy director Viktor Blagov told Itar-Tass that no unexpected "downfall" was likely.
A crew is due to visit Mir in January to prepare the craft for destruction.
Mission control usually attempts to contact Mir once or twice a day. On Tuesday it was doing so every 90 minutes without success.
The Interfax news agency said that even if radio contact cannot be established, an emergency crew could be sent to try to regain control over the station.
Earlier, Russian Aerospace Agency director Yuri Koptev said that a crew of two cosmonauts was getting ready for blastoff in case of an emergency.
But BBC News Online science editor David Whitehouse says that if contact with Mir is lost for an extended period, it will start to spin, and it would become impossible for a rescue team to dock.
Last crew
The decision to scrap Mir was taken because Moscow wants to concentrate its resources on the new International Space Station - a 16-nation joint project that has been subject to repeated delays because of funding problems with Russian modules.
Mir's last crew returned to earth in July.
After surviving a fire and near disastrous collision with an unmanned cargo ship in 1997, followed by a series of computer glitches and breakdowns, Mir had been running relatively smoothly this year.
Efforts to earn the space station a stay of execution by finding private backers ended in failure.
When launched in February 1986 it was seen as a major symbol of the Soviet Union's prestige and technological advancement.