Abstract: In this paper, we present the results of studies of the heat capacity of single-crystal garnets Er _3 ‒ xTm _xAl ₅O ₁₂ (x = 0, 1, 2, 3) in magnetic fields of up to 9 T and in the temperature range 1.9–220 K. The temperature dependences of the heat capacity are approximated using the sum of the contributions of Schottky anomalies associated with magnetic Er ³⁺ and Tm ³⁺ ions and the Debye and Einstein lattice contributions. Entropy and magnetic entropy are calculated using the heat capacity data. With an increase in the magnetic field, entropy is shown to decrease. This indicates the possibility of using the studied garnets in the adiabatic demagnetization method.