We have identified an efficient photochemical transformation involving the reaction of a [1,3]oxazine with tertiary amines. In aerated acetonitrile, the process is accompanied by the appearance of an intense absorption in the visible region. The irreversible formation of 4-nitrophenolate chromophores is responsible for the development of this band. The photogenerated absorption persists indefinitely upon storage in the dark or even under visible illumination. Furthermore, the process is particularly efficient when the [1,3]oxazine is reacted with triethylamine. The corresponding quantum yield is ca. 0.6. Their photoinduced reaction occurs also in poly(methylmethacrylate) matrices. Once again, the process is accompanied by the appearance of an intense absorption and induces pronounced changes in the refractive index of the dye–polymer composite. The irreversible absorption changes associated with these photosensitive materials can be exploited, in principle at least, to write information optically. The stored data can also be read optically relying on fluorescence measurements. Indeed, we have assembled a bilayer structure consisting of a photosensitive film overlaid on a poly(methylmethacrylate) film doped with naphthalene. The fluorescent dopant in the bottom layer emits light in a wavelength range where only the photogenerated state of the top layer absorbs. Thus, the emission coming from one component can be re-absorbed and filtered only by one of the two states of the other. On the basis of this mechanism, a simple fluorescence measurement can read the state of the photosensitive layer, which can, in turn, be written with an optical stimulation.