We have studied by 57Fe Mössbauer spectroscopy and magnetic measurements the spin transition exhibited by the series: [FexCo1-x(4,4′-bis-1,2,4-triazole) 2(NCS)2].H2O, consisting of a spin-crossover iron(II) system diluted in a high-spin cobalt(II) matrix. We performed Monte Carlo simulations of the thermal variation of the high-spin fraction, using a two-level Ising-type Hamiltonian. A good agreement with experiment is obtained by the introduction of an indirect interaction of the spin-crossover units via the non-spin crossover unit, the origin of which is discussed in steric terms. © Les Éditions de Physique 1996.