Preisach-type models were used to analyze static and light induced hysteresis exhibited by spin transition compounds. Magnetic and reflectivity measurements were performed on spin-crossover solid solutions. Analysis suggested that photo-stationary instability due to competition between photo-excitation and non-exponential relaxation of light populated metastable state resulted in light induced thermal hysteresis. Experimental results indicated that frequent defaults obtained from the solutions resulted in inhomogeneous composition of Fe/Co in the crystal.