THE monoclinic unit cell and the space group of NbAs2 and isomorphous TaAs2 have recently been determined by Saini et al.1. In the course of an examination of transition-element polycompounds I observed this structure also in VP2, VAs2, NbP2, TaP2, MoAs2, α-WP2 (low-temperature modification) and WAs2. The titanium-group phosphides TiP2, ZrP2 and HfP2, on the other hand, were found to be isostructural with the orthorhombic ZrAs2 (ref. 2) and HfAs2 (ref. 3). This structure is referred to as PbCl2 (C23) type. However, in these titanium-group pnictides anion–anion bonds seem to be important so that I prefer to distinguish this variety from the ionic PbCl2 type by using a different name. The lattice constants, as derived from powder patterns, are listed in Table 1.