Transient absorption spectroscopy with femtosecond time resolution was used to probe the photochemical ring opening process of 3,3-diphenylnaphtho[2,1-b] pyran in solution. Analysis of the temporal evolution of the spectra from 0.4 ps to 100 ps has allowed us to detect and identify some of the intermediates involved in the photochemical formation of colored photomerocyanine isomers. From these data and those of recent ab initio and semi-empirical calculations on related photochromic systems, a simple mechanistic model was proposed for the ring opening/closing reaction in room temperature solution.