[en] We report the crystal structure of the orthorhombic perovskite LaCo_1/3Mn_2/3O₃ as determined by neutron diffraction from 5–300 K. A high-temperature x-ray diffraction study is also reported from 290–900 K. At temperatures above 570 K, LaCo_1/3Mn_2/3O₃ transforms to a rhombohedral structure with space group $R\stackrel{¯<!--\; ??\; -->}{3}c$. This rhombohedral phase is also observed in the material at high pressure and the crystal structure has been determined by in situ neutron diffraction at 4.7 GPa. Finally, the ferromagnetic behaviour has been determined by magnetometry and the magnetic structure has been determined using low temperature neutron diffraction at ambient pressure. (paper)