Photogeneration of singlet oxygen O-2(a(1) Delta(g)) resulted from photoexcitation at 248 and 266nm of isoprene-oxygen mixtures at elevated oxygen pressure has been investigated. Pulse energy and oxygen pressure dependence of the singlet oxygen yield indicate one-quantum photoexcitation of the encounter complex isoprene-oxygen C5H8-O-2 as a source of O-2(a(1) Delta(g)). Suggested mechanism of O-2(a(1) Delta(g)) photo generation involves direct excitation of complex with simultaneous change of both partner spins like that observed in van der Waals complex C5H8-O-2 [Vidma et al., J. Chem. Phys. 137 (2012) 054305]. Other supposed pathways involve collision-induced photogeneration of Herzberg III state O-2(A'(3) Delta(g)) or triplet isoprene as the intermediates. (C) 2014 Elsevier B.V. All rights reserved.