AbstractWe have studied the rose bengal‐sensitized photooxygenation ofcis‐ andtrans‐polyisoprene model compounds that contain one unit in methanolic solution, characterized the resulting monohydroperoxides, and determined their yield. We have also demonstrated the possibility of a second addition of singlet oxygen on the same unit and have undertaken a kinetic study to determine the rate constants of all the reactions of 4‐methyl