The study of the effects of metal dilution in the solid state on the spin-crossover behavior of [Fe(btr)(2)(NCS)(2)].H2O (btr = 4,4’-bis-1,2,4-triazole), previously carried out on the mixed-crystal series [FexNi1-x(btr)(2)(NCS)(2)].H2O, was completed on the series [FexCo1-x(btr)(2)(NCS)(2)].H2O by using magnetic and calorimetric measurements, as well as Fe-57 Mossbauer-effect spectrometry which provided the possibility of investigating highly diluted samples (down to x approximate to 4.10(-3)). These compounds present interesting peculiar properties : their structure is quasi-two-dimensional and the thermally-induced spin transition they exhibit may occur with a hysteresis effect. When cobalt(II) concentration increases, the iron(II) spin-crossover curves become globally smoother and smoother, and the amount of the residual high-spin form which appears at very low temperature for x less than or equal to 0.4 gets larger. Moreover, a variation of x from 1.00 to 0.23 is found to result in a lowering of the transition temperatures, which pass from 121 to 98 K in the cooling mode and from 145 to 98 K in the heating mode. The hysteresis width, which is 24 K for x = 1.00, vanishes for x approximate to 0.37. The enthalpy (entropy) change associated with the spin conversion of 1 mol of iron(II) mononuclear entities varies from approximate to 10.0 kJ.mol(-1) (approximate to 76.J.K-1.mol(-1)) for x = 1.00 to approximate to 2.0 kJ.mol(-1) (approximate to 17 J.K-1.mol(-1)) for x = 0.23. When comparing the data obtained for the related series [FexCo1-x] and [FexNi1-x], differences are observed concerning the evolution with x of the transition temperatures and of the hysteresis width, as well as the existence of a residual high-spin fraction at low temperatures. The above findings were accounted for qualitatively by taking into account the relative sizes of Co(II), high-spin Fe(II), and low-spin Fe(II) ions and then quantitatively on the basis of a thermodynamic model.