Dihydroindolizines (DHI) are photochromic compounds, which on irradiation with long wavelength UV or visible light afford a colored open photoisomer, the betaine. Light induces breaking of the azacyclopentene ring. In the dark, there is only one thermal back reaction (ring closure) The determination of photochromic parameters of these systems such as the quantum yields and the UV/visible spectra of the unstable photoisomers is not a trivial problem and requires appropriated methods. In this paper, we show that kinetic modelling and inverse treatment of experimental (Abs vs t) plots recorded under continuous irradiation can help to solve this problem (dynamic method).