The formation of singlet oxygen O-1(2) provided by the photoexcitation of the encounter complexes of isoprene with oxygen (C5H8-O-2) in the gas phase within the spectral region 253.5-355 nm has been observed at the elevated pressure of oxygen. Singlet oxygen has been detected with its NIR luminescence centered near 1.27 mu m. The photogeneration of O-1(2) is found to be a one-photon process. In the UV-C region (253-278 nm) the quantum yield of O-1(2) is measured. This yield of O-1(2) is governed mainly by photoexcitation of O-2 molecules to the Herzberg III ((3)Delta(u)) state via enhanced absorption by C5H8-O-2 collision complexes. So excited triplet O-2 gives rise to singlet oxygen because of triplet-triplet annihilation in the collisions with unexcited O-2 molecules. In the UV-B (308 nm) region the appearance of O-1(2) is attributed to the excitation of a double spin-flip (DSF) transition in complex C5H8-O-2. In the UV-A region (355 nm) besides DSF the O-2-assisted T-1 <- S-0 excitation of isoprene to the triplet state takes place, which is a sensitizer of O-1(2) formation. The contribution of the encounter complexes C5H8-O-2 to the production of singlet oxygen and to the lifetime of isoprene in the Earth's troposphere are estimated.