Abstract Dilute solutions (2 × 10 −3 M) of poly( para-propionylstyrene) were exposed to long-wave u.v. radiation ( λ ⩾ 300 nm) under high vacuum at 25°C. Methane and ethane were formed, their quantum yields [< 10 −4 mol(einstein) −1] being lower than those observed for the solid state photolysis. Both α-cleavage (Norrish Type I) and β-fission of the ethyl group are involved. Chain scission occurs more readily in solution; this effect was attributed to the decomposition (β-scission) of the tertiary radicals formed during photoreduction. Cyclization involving non-neighbouring tertiary radicals and cross-linking also occur. The presence of H-donors (isopropanol) reduces the extent of chain scission, and molecular weight increases due to cross-linking are then observed. Solvent quality plays an important role, the poorer solvent leading to a more tightly coiled polymer, in which intra-coil photoreduction is facilitated, and the rate of chain scission is increased.