An electron microscopic study of the Co3V alloy microstructure formed after heat treatment at different temperatures has been conducted. Two phase transitions ordering–phase separation have been discovered, which occur in the alloy at temperatures of about 450 and 800°C. At high-temperature phase separation, the microstructure consists of bcc vanadium atom particles and a fcc solid solution; at low-temperature phase separation, it is a cellular structure, in which the cellular boundaries are enriched in vanadium. In the region of ordering, it consists of particles of the Со3V chemical compound, randomly distributed in the solid solution. It is shown that at none of the temperatures do these microstructures correspond to those shown in the Со–V phase diagram.