The kinetics of spiropyran photochromic systems in solution in a stirred batch reactor continuously irradiated with monochromatic light was studied by UV/visible spectrophotometry. The plots of absorbance vs time were analyzed, and the desired parameters (quantum yields, UV/visible spectrum of the unstable photomerocyanine, ...) were extracted from an iterative computation which fitted the calculated curves to the experimental ones on the basis of a representative model of the reaction mechanism. The method was applied to spiro-[benzothiazoline-2’,2-benzopyran] whose corresponding photomerocyanine has a lifetime of 100 s in toluene; the two quantum yields of the direct process of photocoloration and the reverse reaction of photobleaching could be determined along with the spectrum of the corresponding photomerocyanine. To demonstrate the general nature of the photokinetic method, it was applied to the reaction of spiro[indoline-2’,2-benzopyran], where the main photochromic process is accompanied by a photodegradation. Despite this interfering phenomenon, the photokinetic method could be used to extract the parameters of the main photochromic process. It also showed that the photodegradation products catalyzed the thermal back-isomerization. The order of magnitude of the rate constant of this catalytic process and the quantum yields of photodegradation were estimated.