This paper brings together the results of our recent studies on various aspects of UV initiated solid-state polymerizations of acrylic and methacrylic acids. The use of polarized UV radiation to initiate the polymerization of acrylic acid at 4°C is consistent with the initial formation of mobile excitons which are subsequently trapped at the imperfections where radicals are eventually formed. Differences in the electron spin resonance spectra and radical concentrations obtained by irradiating samples of methacrylic acid under various conditions in the temperature range − 5O°C to − 20°C demonstrate the existence of a range of imperfections in the crystalline monomer, the nature of which is sensitive to the presence of trace impurities. The molecular mobility in the imperfections is seen to increase with increasing temperature in a manner consistent with broad-line n.m.r. studies on the monomer. These results give information on the mechanisms of initiation and termination. The effect of small applied stresses on the polymerizations is sensitive to both temperature and trace impurities; i.e., factors which affect molecular mobility in the bulkmaterial and in imperfections. Themechanism of the polymerization of crystalline acrylic and methacrylic acids is discussed in terms of these results.