In films of Poly(para-phenylphenylenevinylene) a photoreaction has been observed. The significant feature is a decrease of the fluorescence emission intensity during irradiation. It is assumed that a cycloaddition between C=C- double-bonds leads to crosslinks in the material. Under similar conditions of excitation a photoreaction has not been observed in poly(p-phenylenevinylene) prepared via the precursor-route.