[en] Investigation into ErNi interaction with hydrogen in the pressure region of < 1 MPa and at temperatures close to room ones is conducted using a differential automatic calorimeter. X -ray phase analysis of the reaction products allows one to consider that transformation of the source FeB structure in CrB takes place. Elementary cell parameters for ErNiH_3.6 are determined: a=0.3655 (3), b=1.1200 (8), c=0.4590 (5) nm. Dependence of differential enthalpy of hydrogen dissolution in ErNiH_x on x is studied. It is shown that in the region of 0 < x 1 < 1.0 and 1.0 < x < 3.0 compositions the differential dissolution enthalpies have constant values equalling respectively to -99.1 ± 0.7 and -90.6 ± 0.7 kJ/mol of H₂ErNi hydration enthalpy up to ErNiH_3,0 composition, equalling to -93 ± 1 kJ/mol of H₂ is calculated by dissolution differential enthalpy integration