Abstract The photo-oxidation, at long wavelengths ( λ > 300 nm), of polyisoprenes (IR) with different compositions ( i.e. with different relative concentrations of 1,4 cis, 3,4 and 1,2 groups) have been compared. Abstraction of a hydrogen atom in the allylic position is probably the main oxidation pathway of the 1,4 cis unit. The tertiary macroradical formed is oxidized to tertiary associated hydroperoxides. Ketonic groups (α,β unsaturated and saturated) are detected in the IR spectra of photo-oxidized IR at 1693 and 1726 cm −1, respectively. Formation of ketonic groups induced β cleavage and numerous chain-scissions. Viscosity and GPC measurements reveal a decrease in the weight-average molecular weight during the earliest stages of irradiation. Other photo-products are detected: The major ones are alcohols together with acids, esters and epoxides. An addition mechanism occurs and competes with chain scission during irradiation. In the case of 1,2 and 3,4 units, radical addition is probably the main oxidation pathway.