The quenching rate constants of singlet oxygen O-2((1)Delta(g)) luminescence at 1.27 mu m by 29 amines and aromatic hydrocarbons were measured in acetonitrile and benzene. Quenching occurs via reversible charge transfer with the formation of an exciplex with a partial (delta approximate to 0.2 esu) electron transfer. An estimate of the intersystem-crossing rate constant for the exciplex of ca. 10(10) s(-1) is obtained, and a modest decrease with increasing exciplex energy is noted. The data predict and it is confirmed that charge transfer plays a significant role in the nonradiative decay of singlet oxygen in solvents as difficult to oxidize as toluene-d(8) and mesitylene. It is also confirmed that N,N,N＇,N＇-tetramethyl-p-phenylene diamine quenches O-2((1)Delta(g)) nearly exclusively by full electron transfer in D2O, but electron transfer is not observed in other solvents.