We demonstrate spontaneous desorption time of flight mass spectrometry investigations on chemical reactions within thin solid films with a thickness of a few monolayers deposited by the Langmuir-Blodgett-Kuhn (LBK) technique and in the mum regime by spincoating. The photo induced decomposition of the polymeric system of a dye, disperse red 1, covalently bound to polymethylmethacrylate is one of the systems under investigation. Films of a thickness, which are typical for channel waveguides, were illuminated with UV and annealed subsequently. The decomposition of the dye after the illumination could be proven by the decrease of the mass peaks of intact dye and the increase of dye fragments. The annealing process leads to an evaporation of the loosened dye, its fragments and fragments of the polymer backbone. The results are compared with in situ bleaching experiments observed by time dependent photo thermal beam deflection. The second system consists of a poly [(tert-butylacrylate)-co-CN(3-methacroylpropyl)-3,4-dimethylmaleicacidanhydride)] multilayer LBK sample that is cross-linked via UV irradiation and saponified by wet HCl treatment to achieve a thin film poly (methacrylic acid) network. The time development of the saponification is observed by the mass spectrometry and Fourier Transform Infrared Spectroscopy.