Thermodynamic properties of Mg(NH2)(2) and LiNH2 were investigated by measurements of NH3 pressure-composition isotherms (PCI). Van't Hoff plot of plateau pressures of PCI for decomposition of Mg(NH2)(2) indicated the standard enthalpy and entropy change of the reactions were Delta H degrees = (120 +/- 11) kJ . mol(-1) (per unit amount of NH3) and Delta S degrees= (182 +/- 19) J . mol(-1) . K-1 for the reaction: Mg(NH2)(2) -> MgNH +NH3, and Delta H degrees= 112 kJ . mol(-1) and Delta S degrees = 157 J . mol(-1) K-1 for the reaction: MgNH -> (1/3)Mg3N2 + (1/3)NH3. PCI measurements for formation of LiNH2 were carried out, and temperature dependence of plateau pressures indicated Delta H degrees = (-108 +/- 15) kJ . mol(-1) and Delta S degrees = (-143 +/- 25) J. mol(-1) . K-1 for the reaction: Li2NH + NH3 -> 2LiNH(2).