Based on the literature data of the enthalpies of solution in water,Δ_solH^θ_m, lattice enthalpy,ΔH^θ_L,lattice energy,ΔU^θ_L for the complexes of the lanthanidi metals with 3-nitro-1,2,4-triazol-5-one (NTO),M(NTO)_n·mH₂O(M=La,Ce,Pr,Eu,Sm,Gd,n=3,m=7; M=Y,Yb,n=3,m=6; M=Dy,Tb,n=3,m=5; M=Nd,n=3,m=8),standard enthalpies of formation,Δ_fH^θ_m(Mⁿ⁺,aq,∞),Δ_fH^θ_m(Mⁿ⁺,g),Δ_fH^θ_m(H₂O,g),Δ_fH^θ_m(H₂O,l),Δ_fH^θ_m(NTO^-,aq,∞),Δ_fH^θ_m(NTO^-,g) and hydrous enthalpy of Mⁿ⁺,Δ_hH^θ_m(Mⁿ⁺),the hydrous enthalpy of NTO^-, Δ_hH^θ_m(NTO^-) was estimated as －(153.73±0.21) kJ·mol^-1.