Almtract--The kinetics have been studied quantitatively and a mechanism proposed for the free-radical degradation of solid PMMA initiated by photoreduction of the chloride complexes of Fe 3+. The polymer was exposed in an inert atmosphere at 77 and 293 K to light of various spectral compositions with 2 > 300 nm, not absorbed by the polymer. The rate of the build-up of the double isolated and conjugated bonds, formed as a result of thermal transformations of the PMMA macroradicals at 293 K, has been studied. It is shown that the isolated double bonds are generated at a rate equal to the initiation rate W~ and the conjugated double bonds are generated at the rate of 0.3 W,. A mechanism is proposed closely describing the observed regularities. It has been found that the degradation of PMMA irradiated at 77 K results from the thermal decomposition of macroradicals on heating the samples and is dependent upon lhe spectral composition of the light. The probability of degradation is 0.16 per photoreduced Fe 3-ion for light 2 < 370 nm and decreases to only 0.009 for light with 2 > 390 nm. It is concluded that macrochain breaking under these conditions is due to the thermal decomposition of the macroradicals ~CH2(~(CH3)CH2~. At 293 K the photoinitiated PMMA degradation is a free-radical, but not a chain, process independent of the intensity and spectral composition of the light (in the wavelength range 313-390 nm), molecular mass of the polymer and film thickness. Degradation in an inert atmosphere is characterised by a probability factor per photoreduced Fe 3+ ion (~) which increases with the degree of conversion of the initiators. The rate of degradation in an atmosphere of HC1 is directly proportional to the initiation rate W~. It is concluded that, at 293 K in an enert atmosphere, the rupture of macromolecules is due to the thermal decomposition of both the primary macroradicals ~C(CH3)(COOCH3)12H-C(CH3)(COOCH3)~ and the radicals ~CH2-~(CH3)CH2~ formed by addition of low-molecular radicals to the radical reaction products in this system, i.e. the isolated middle double bonds. Pi~ol/C o and Pco,j./C o in PMMA (I) and the initiator conversion degree AC/Co: (1 and 3) Pi~ol ; (2 and 4) Peon) ; (1 and 2) [Y]0 = 0.05 mol/kg, /= 9 #; (3 and 4) [Y]0 = 0.14 mol/kg, l = 1/~, 2 = 365 nm, 293 K.