The paper reports on kinetic measurements of coloring and bleaching reactions in the spiropyran – merocyanine photochromic system dissolved in poly(methyl methacrylate) matrix (PMMA/SP). The system was earlier used as the gate insulator in a photo-switchable organic field effect transistor (OFET), and the switching was attributed to the photochromic reaction taking place in the polymer matrix. The present measurements aimed at elucidating the mechanism of the switching by comparing the rate constants of the coloring and bleaching reactions with the rate constants of processes responsible for capacitance changes. The comparison of the rate constants seems to confirm the role of the photochromic process: to within experimental uncertainty the rate constants determined from the two methods are similar. Measurements of the temporal evolution of absorbance additionally reveal existence of an irreversible fatigue reaction, probably associated with admission of oxygen to some samples. The presence of oxygen is also responsible for a deterioration of performance of the OFETs under study.