SynopsisPoly( methyl methacrylate) (PMMA) was photolyzed with monochromatic light of wavelengths 260, 280, and 300 nm by the use of the Okazaki large spectrograph. The quantum yield of mainchain scission ( aC8), effects of wavelength, and incident photon intensity on the photodegradation were investigated. It turned out that photodegradation of PMMA took place by the irradiation of 260-300 nm light, but did not by the irradiation at X 2 320 nm. The number of main-chain scission ( N c s ) has a maximum value in the case of the irradiation with 280 nm light. Under the same conditions of wavelength and total photon intensity, a longer-term irradiation with a weaker incident photon intensity tends to yield a greater amount of main-chain scission. A linear relationship was found between the number of main-chain scission and the incident photon intensity. The average values of aC8 obtained in this work were 2.1 X 2.4 X and 4.1 X respectively, for the irradiations a t 260, 280, and 300 nm. It was found that the photoinduced side-chain scission initiates the main-chain scission of PMMA.