Poly-ct-methylstyrene (PctMS) was degraded in CHCI 3 and CC14 solution by flash photolysis (2 = 265 nm). The degradation, as detected by light scattering measurements, is caused by the attack on PctMS by solvent radicals, assumed to be formed mainly by energy transfer processes. The direct effect did not lead to detectable main chain cleavage as evidenced by experiments with PctMS dissolved in dioxane or methylene chloride. The time dependence of the decrease of the light scattering intensity (LSI) after the flash was measured. The observed first order lifetime z(LSI) corresponds to the lifetime of lateral macroradicals P' that decompose by main chain cleavage (k = 3.5 x 10 2 sec-1). Molecular oxygen reacts with the lateral macroradicals with k = (5.5 + 0.5) 10 5 M 1 sec 1. Only a minor portion of the product of this reaction (PO;) decays by main chain scission. Thus O2 inhibits main chain scission significantly. By addition of cyclohexane and ethyl mercaptan, the main chain cleavage is inhibited, r(LSI) was not affected by these compounds in the concentration range investigated ([C6H12]: up to 8.4M; [C2HsSH]: up to 3 x 10 -3 M), indicating that they reacted with solvent radicals which otherwise attack the polymer forming lateral macroradicals.