[en] Highlights: • Phase diagram was first established in Co₂Cr(Ga_1−xSi_x) Heusler alloys. • Existence region of the single phase in the L2₁-type was confirmed in x ⩽ 0.5. • Magnetic measurements predict Co₂Cr(Ga,Si) are half-metallic ferromagnets if the order degree is completely controlled. -- Abstract: The phase diagram, magnetic properties and the region over which the Heusler alloys Co₂Cr(Ga_1−xSi_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 L2₁ to the B2 phase, T_t^L21/B2, increased almost linearly with increasing x. The value of T_t^L21/B2 for the L2₁-type Co₂CrSi Heusler alloy, estimated by linear extrapolation from its concentration dependence, was about 1450 K, and therefore higher than that of Co₂CrAl and Co₂CrGa. The Curie temperature, T_C, also increased with increasing x becoming 600 K 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 5 K increased with increasing x, almost following the generalized Slater–Pauling (S.P.) rule predicted by Galanakis et al