Measurements of three high-spin FeIIS4 complexes at various temperatures and applied magnetic fields are reported and the results are compared with those of reduced rubredoxin. Simulations of the measured spectra by means of the fully developed stochastic theory of lineshape prove that intermediate relaxation behavior is present and is determined essentially by the phonon energy distribution and by one single parameter W0, the scaling factor, which contains the strength of spin-lattice coupling.