The photogeneration of singlet oxygen O-2(a(1)Delta(g)) resulted from the photoexcitation at 248 nm of gaseous oxygen at elevated pressure (3-80 atm) has been investigated. Singlet oxygen monitored by its IR-luminescence at 1.27 mu m was found to appear via two processes which are identified to be due to absorption of individual O-2 molecules and encounter complexes O-2-O-2, respectively. The one-quantum absorption of complexes gives rise to appearance of oxygen in Herzberg III state O-2((3)Delta(u)) which is supposed to be a triplet sensitizer responsible for singlet oxygen O-2(a(1)Delta(g)) production observed in our experiments. (C) 2009 Elsevier B. V. All rights reserved.