Quenching of singlet-oxygen-sensitized 703, nm delayed fluorescence (SOSDF) of tetra-tert-butylphthalocyanine (Pc) by beta-carotene, a-tocopherol, 1,4-diazabicyclo [2.2.2]octane, 2,6-di-tert-butyl-4-methylphenol, and lauric acid, compounds whose singlet oxygen quenching rates vary by 6 orders of magnitude, was found to be caused entirely by singlet oxygen quenching. The apparent rate constants for quenching of SOSDF are twice those for quenching of singlet oxygen 1279 nm phosphorescence under all conditions, as required by the fact that two molecules of O-1(2) are needed. Quenching of the visible SOSDF provides a highly sensitive method for measurement of rate constants for O-1(2) quenching, which will be usable with commonly available apparatus or in systems where the 1270 nm luminescence is difficult to detect.