[en] The effect of the substitution of Co for Fe on the formation, structure and magnetic properties of Sm₂Fe₁₆GaC_y compounds were studied. Compounds with composition Sm₂(Fe_1-xCo_x)₁₆GaC (x=0-0.3) and Sm₂(Fe_0.8Co_0.2)₁₆GaC_y (y=0-2.0) were prepared by arc melting. X-ray diffraction shows that these carbides are almost single phase with the rhombohedral Th₂Zn₁₇-type structure. The unit cell volume (v) decreases monotonically with increasing Co concentration (x). The Curie temperature (T_c) increases with x from 592 K for x=0 to 816 K for x=0.3. The room-temperature saturation magnetization and the anisotropy field are 126.4 emu g^-1 and 117 kOe respectively when the Co concentration x reaches 0.3. Addition of carbon results in the decrease of saturation magnetization and has a small effect on the Curie temperature and anisotropy field. (orig.)