The phase diagram, magnetic properties and the region over which the Heusler alloys Co 2 Cr(Ga 1 − x Si x ) occur as a single phase have been established. A single phase was obtained in the composition range of x⩽0.5, in which the order–disorder phase transition temperature from the L2 1 to the B2 phase, TtL21/B2, increased almost linearly with increasing x. The value of TtL21/B2 for the L2 1 -type Co 2 CrSi Heusler alloy, estimated by linear extrapolation from its concentration dependence, was about 1450K, and therefore higher than that of Co 2 CrAl and Co 2 CrGa. The Curie temperature, T C , also increased with increasing x becoming 600K at x=0.5, thus reflecting an increase in the magnetic moment caused by the change in the number of the valence electrons. The concentration dependence of the spontaneous magnetic moment, M s , measured at 5K increased with increasing x, almost following the generalized Slater–Pauling (S.P.) rule predicted by Galanakis et al.