From measurements of the enthalpy of solution of metal salts of 3-nitro-1,2,4-triazol-5-one (NTO) in water, the standard enthalpies of formation of KNTO·H2O, Ba(NTO)2·3H2O, LiNTO·2H2O, Ca(NTO)2·4H2O and Gd(NTO)3·7H2O were determined as −(676.9±2.6), −(1627.0±2.5), −(966.6.3±2.2), −(1905.5±4.4) and −(3020.1±6.4) kJ·mol−1, respectively. From measurements of the enthalpy of precipitation of KNTO·H2O crystal with Pb(NO3)2(aq), CuSO4(aq) and Zn(NO3)2(aq), the standard enthalpies of formation of Pb(NTO)2·H2O, Cu(NTO)2·2H2O and Zn(NTO)2·H2O were determined as −(247.4±5.9), −(712.1±5.4) and −(628.8±5.7) kJ·mol−1, respectively.