In the [Fe(mtz)(6)] (BF4)(2) spin-crossover system, the Fe(II) complexes occupy two nonequivalent lattice sites. Complexes on lattice site A show a thermal high-spin (HS) --> low-spin (LS) transition at 74 K, whereas complexes on site B remain in the high-spin state down to 10 K. Complexes on both sites exhibit light-induced spin-state conversions at 20 K : LS --> HS on site A after irradiation at 514.5 nm; HS --> LS on site B after irradiation at 830 nm. The subsequent relaxation processes are strongly influenced by cooperative effects of elastic origin, leading to characteristic deviations from first-order kinetics. This is attributed to a buildup of an internal pressure during the relaxation process, giving rise to a shift in both the horizontal (bond length difference Delta r(HL)) and the vertical (energy gap Delta E(HL)(O)) displacements of the potential wells of the two states relative to each other. The shifts can be estimated from the observed shift of the d-d absorption bands on the order of 100 cm(-1). For site B complexes, this may lead to a change in the true ground state.