X-ray diffraction studies on the Cu1-xCoxCr2Se4 system (x = 0, 0.2, 0.4, 0.6, 0.8, 1) proved the existence of the cubic spinel-type structure for x <= 0.2 and the monoclinic Cr3S4-type one for x >= 0.8. CoCr2Se4 undergoes an order-disorder phase transition above 750 degrees C. The high-temperature CoCr(2)Se4 polymorph crystallizes in the filled Gdl(2)-type structure with the P (3) over bar m1 space group. The trigonal-to-monoclinic symmetry breaking in CoCr2Se4 is found to be first-order within Landau theory. Cc distribution in the low-temperature monoclinic CoCr2Se4 structure depends on cooling conditions of trigonal CoCr2Se4: rapid quenching results in Co disorder in the monoclinic structure, while slow cooling yields a fully ordered monoclinic structure.