The photofragmentation reaction of poly(methylmethacrylate) (PMMA) by soft x-ray irradiation was investigated by measuring the yield of released neutral fragments and the changes in the absorption spectra in the carbon and oxygen K-edge regions. Films of 0.4-1.0 mu m thicknesses were irradiated with various doses of monochromatic 300, 400, and 600 eV soft x-rays. The effect of irradiation was monitored by the yield of the fragment products released from PMMA, as a function of dose. The fragments of mass 15, 28, 29, 31, 44, and 60 amu, corresponding to CH3, CO, CHO, CH3O, CO2, and C2H4O2, were chosen for monitoring this irradiation dose dependence. Each fragment showed a distinct response to irradiation. The etching rates, represented by the yield of the dependent on the original thickness of the sample and gradually decrease with dose. From the analysis of the thickness and dose dependencies, we concluded that secondary reactions in the bulk play an important role in the desorption mechanism. The absorption spectrum after irradiation indicates the abstraction of the ester group and the formation of double bonds. All the observations are in agreement with a degradation mechanism involving the ester group abstraction followed by main chain scission and, at large doses, crosslinking and the formation of a protecting overlayer of chemical nature similar to poly(butadiene).