Crystalline acrylic and methacrylic acids can be polymerized by ultra-violet irradiation at temperatures not far removed from their melting points. In both cases the reaction proceeds in such a way as to give non-crystalline and disoriented polymer. Irradiation of methacrylic acid under these conditions gives a measurable free radical concentration, whereas no free radicals are detectable by electron spin resonance in irradiated acrylic acid. The production of observable free radicals in methacrylic acid crystals is very temperature dependent, decreasing with reduction in temperature. The polymerizations have been studied in some detail by observing the changes in the optical retardation of thin single crystals while undergoing reaction. In this way a post-irradiation reaction was observed with methacrylic acid but not with acrylic acid samples. The most striking observations were those obtained when the crystals were subjected to small mechanical compressive stresses, when the polymerization reaction was generally retarded or stopped. To account for the results obtained it is suggested that the polymerization reaction takes place in association with crystal dislocations. The effect of mechanical stress is considered to arise from the displacement of dislocations. Two possible mechanisms, depending on the properties of crystal dislocations, have been suggested. Although more experimental data are required it is thought that the observed pressure effect will be of considerable importance in the elucidation of the details of the polymerization.