The effects of metal dilution on the spin-crossover behavior of [Fe(btr)2(NCS)2].H2O (btr = 4,4’-bis(1,2,4-triazole)) have been studied in the mixed-crystal series [FexNi1-x(btr)2(NCS)2].H2O, using magnetic and calorimetric measurements. These compounds have a two-dimensional structure, and the thermally-induced spin-crossover process may present a hysteresis effect. Iron(II) dilution tends to smooth the transition curves. Moreover, when x changes from 1.00 to 0.26, the transition temperature varies from 121 to 132 K in the cooling mode and from 145 to 132 K in the heating mode. The hysteresis, which is 24 K for x = 1.00, vanishes for x almost-equal-to 0.45. No residual low-spin (high-spin) fraction is observed at room (very low) temperature. The enthalpy (entropy) change associated with the spin conversion of 1 mol of iron(II) mononuclear complex decreases from 10.2 kJ.mol-1 (76.4 J.mol-1.K-1) for x = 1.00 to 3.6 kJ.mol-1 (27.5 J-mol-1.K-1) for x = 0.26. These effects were interpreted first qualitatively, and then quantitatively, on the basis of a thermodynamic model, taking into account the relative sizes of Ni(II), high-spin Fe(II), and low-spin Fe(II) ions.