Launch from the Baikonur Cosmodrome; landing 200 km southwest of Kokchetav.

Following a one day solo flight Soyuz 21 docked with Salyut 5 on July 07, 1976. The docking was problematic, because of a failure of the Igla system.

Their stay coincided with the start of the Siber military exercise in Siberia, which they observed as part of an assessment of the station’s military surveillance capabilities. They conducted only a few scientific experiments, including the first use of the Kristall furnace for crystal growth. Engineering experiments included propellant transfer system tests with implications for future operation of the freight-carrying Progress spacecraft.

Observation of aquarium fishes in the microgravity were done too. Solar observation and biological observation (plant growing) were carried out. The Kalialaun-experiment was needed for the observation of crystal grooving in space. Also a TV conference with pupils was held.

A premature return to Earth was needed because of suspected toxic gases in the space station. Vitali Zholobov suffered from space sickness and homesickness and the crew from psychological problems. At the landing date the crew was in very bad psychological and mental condition. So it could be, that the problem with toxic gases could have been only a pretext, but nothing is sure.

The cosmonauts boarded Soyuz 21 but as Boris Volynov tried to undock from the station, the docking latches failed to release properly. As he fired the jets to move the spacecraft away, the docking mechanism jammed, resulting in the Soyuz being undocked but still linked to Salyut. As the two spacecraft moved out of range of ground communications, the cosmonauts received only the first set of emergency procedures. Boris Volynov tried a second time to undock but only managed slightly to loosen the latches. The situation persisted for an entire orbit, 90 minutes, when the final set of emergency procedures were received and the crew finally disengaged the latches.

Because Soyuz 21 was returning early, it was outside the normal recovery window. It then encountered strong winds as it descended, which caused uneven firing of the retrorockets. It made a hard landing around midnight.