In the [Fe(mtz)6] (BF4)zspin-crossover system, the Fe(I1) 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 AQL) and the vertical (energy gap A,?$,,) 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-I. For site B complexes, this may lead to a change in the true ground state.