This paper focuses on waveguide technique versatility for direct observation of photochemical reactions in polymer ultrathin films. Monolayers from the copolymer of 9-anthrylmethyl methacrylate (AMMA) with neopentyl methacrylamide (nPMA) were deposited on a tapered quartz waveguide by the Langmuir-Blodgett (LB) technique, and spectroscopic properties of the copolymer (p(nPMA-AMMA)) monolayer were precisely monitored by an integrated optical waveguide technique. Magnitude of absorbance increased by a factor of 200, compared with transmission W-vis spectroscopy. Consequently, a photo-cross-linking reaction of p(nPMA-AMMA) LB films was characterized easily and very accurately as a function of irradiation time.