We study both theoretically and experimentally kinetics of photoinduced ordering in azo-dye films. From the absorption-vs-incidence angle curves measured for the probe light linearly polarized parallel and perpendicular to the plane of incidence we obtain dependence of the principal extinction coefficients on the irradiation dose. The transient photoinduced structures are found to be biaxial. The measured irradiation time dependence of the absorption order parameters is interpreted by using the generalized diffusion model.