On exposure to violet and ultra-violet radiation, crystals of salicyIidene-m-toluidine and of salicylidene-aniline both develop an absorption band with Amax = 4750A. A theory is developed which permits determination of the quantum efficiencies of this reaction (8 and 0.04-049 respectively). This band bleaches in darkness; the rates of bleaching are determined at various temperatures, and activation energies for the bleaching reaction are calculated : 1.28 eV and 0.99-1.65 eV respectively ; the behaviour of S.A. is not simple and the observations are attributed to the occurrence of two crystalline forms having different rates of colouring and bleaching.The bleaching of each compound is accelerated by irradiation within the induced band, and quantum efficiencies of this optical bleaching are determined for A = 5461 A as 0.04 and 0.2 respectively. Oscillator strengths for the induced absorption band are determined (-10-3). It is suggested that the colouring process is due to the intramolecular transfer of an H atom during the bending vibration of the C-CH=N group in the electroaically excited molecule. The crystal angles of salicylidene-m-toluidine are reported.The phenomenon of phototropy, photochromism 1 or photochromotropy,2 whereby compounds exhibit a reversible change of colour on exposure to electromagnetic radiation of suitable frequency, has been studied by many workers. Review papers 3-5 list compounds which exhibit this property. Usually the colour change occurs in the solid state, but under certain conditions in solution.2The phototropic change may be represented by the reaction, h v A'A'. The reverse reaction normally takes place in the absence of radiation and is accelerated by raising the temperature (thermal bleaching). In some cases, however, if the coloured material is irradiated with radiation lying within the induced absorption band, the reverse reaction is accelerated (optical bleaching).Information on the nature of the mechanism may be obtained by measuring the differing physical properties of the material in the states A and A'; e.g., dielectric constant,6 magnetic susceptibility,7 X-ray diffraction 6, 8 and optical absorption spectra.9~ 10 This paper describes the apphation of spectrophotometric techniques to determine the reaction kinetics for two anils-salicylidene-m-toluidine (S.M.T.) and salicylidene-aniline (S.A.). EXPERIMENTALMeasurements were made using a quartz prism monochromator on (i) the absorption spectra in states A and A' with the E vector of the measuring radiation parallel to the principal * now at College of Technology, Letchworth.