Differential scanning calorimetry and x‐ray diffractometry have been used to monitor reactions of the C15 Laves phase GdCo₂ during constant‐rate heating in a hydrogen atmosphere and to determine the enthalpy change of each reaction. The values of the enthalpy change ΔH of hydrogen absorption, hydrogen‐induced amorphization and crystallization are evaluated to be −67.2, −10.2, and −9.8 kJ/mol Gd, respectively. On the other hand, ΔH of hydrogen desorption is +31.4 kJ/mol Gd. The amorphous a‐GdCo₂H_x alloy crystallizes to GdH₂ and Co. Therefore, the total value of ΔH of the above reactions along with the formation of GdCo₂, −103 kJ/mol Gd, becomes nearly equal to that of formation of GdH₂, −101 kJ/mol Gd.