The formation of conjugated double bonds in polyvinylchloride (PVC) films by ultraviolet (UV) irradiation is analyzed using an in situ laser-monitoring technique we have developed to measure changes in transmittance of thin UV-irradiated PVC films. Conjugated polyenes of different lengths are produced by laser irradiation at 193 and 248-nm wavelengths. This technique promises to have broader applications in the study of polymer reaction mechanisms and kinetics.