The photochemical decomposition of substituted 1,5-diaryl-3-alkyl-pentazadiene monomers and polymers induced by UV irradiation with a pulsed excimer laser was monitored by UV-vis spectroscopy. The net photochemical reaction proceeds in an overall one-step pathway A-->B. Quantum yields for the laser decomposition are determined to be less than or equal to 10%. An oscillating behavior of the absorption was found during the dark period following the irradiation. The temperature dependence of this dark reaction has been studied. The oscillatory behavior is discussed in the light of current theories concerning instabilities in photochemical systems.