AbstractNiobium and tantalum diselenides have been found to exist in the following polymorphous forms: 2s‐NbSe2, 3s‐NbSe2, 4s‐NbSe2, 2s‐TaSe2, 3s‐TaSe2, 4s‐TaSe2, and 6s‐TaSe2. Unit‐cell dimensions are collected in Table I and the crystal structures of the various forms are schematically shown in Fig. 1. All the polymorphs have hexagonal or rhombohedral layer structures. In the 2s‐, 3s‐ and 4s‐ phases the metal atoms have a trigonal‐prismatic environment, but in 6s‐TaSe2 half of the metal atoms have octahedral and half have trigonal‐prismatic coordination. The conditions of preparation of the various forms are discussed. Excess of selenium over the composition MSe2 leads to mixtures of the MSe3 and MSe2 phases. Excess of metal results in the formation of phases of the type M1+xSe2, where the additional metal atoms are accommodated in the octahedral interstices between the MSe2 slabs.