A study of the magnetocaloric effect for Tm2Fe16, Tm2Fe17, Tm2Fe18, Tm2Fe19, Tm2Fe17−xMnx (x ≤ 1.5), Ce2Fe17−xMnxHy (x < 2, y ≤ 3), Lu2Fe17−xMnx (x ≤ 2) systems at magnetic ordering temperatures TC, TN and a ferro-antiferromagnet phase transition ΘT, from magnetization measurements. In alloys with ground ferromagnetic and high-temperature antiferromagnetic states, the change in the magnetic entropy peak −ΔSM increases as the magnetic phase transition temperatures ΘT and TN converge with varying compound composition. Extrapolating the difference TN− ΘT to zero for the initial and doped alloys allows us to determine the maximum possible value of −ΔSM in such systems and the composition of the corresponding alloy. A superposition of the −ΔSM(T) maxima at ΘT and TN for compounds with two magnetic phase transitions creates much higher values of relative cooling power in comparison to compounds with only one magnetic phase transition at TC.