LB-films of an amphiphile containing diphenyldiacetylene moieties as chromophore have been prepared. Compared to the solution the LB-films show a different intensity ratio between the two diacetylene bands in the UV/Vis spectrum due to the dense packing of the chromophores. Irradiation with polarized light at 360 nm causes an angle dependent increase in the absorbance in the π-band region, which previously has been interpreted to be due to a photoreorientation process. However, a decrease of the π′-band in the UV/Vis spectra and the C≡C vibration band in the grazing angle reflectance IR-spectra indicate a photoreaction of the chromophores, which nature remains elusive. The increase in absorbance in the π-band region is conjectured to be to partial reversal of the intensity ratio changes due to a less dense packing of intact chromophores. From experiments, in which the irradiated LB-film is dissolved, the amount of reacted chromophores can be estimated to less than 20%.