The spin trap 5,5-dimethyl-1-pyrroline N-oxide (DMPO) is frequently used to identify free radicals that are generated photochemically using dyes as photosensitizers. When oxygen is present in such systems, singlet oxygen (O-1(2)) may be produced and can react with DMPO. We have studied the reaction of DMPO with O-1(2) in aqueous solutions over a wide range of pH, using micellar Rose Bengal (pH 2-13) and anthrapyrazole (pH < 2) as photosensitizers. We found that DMPO quenches O-1(2) phosphorescence (k(q) = 1.2 x 10(6) M(-1) s(-1)), thereby initiating oxygen consumption that is slow at pH 10 but increases about 10-fold at pH < 6. This oxygen consumption is a composite process that includes efficient oxidation of both DMPO and its degradation products. The oxidation products include both products in which the DMPO pyrroline ring remains intact (DMPO/(OH)-O-. and 5,5-dimethyl-2-oxo-pyrroline-1-oxyl (DMPOX) radicals) and those in which it becomes opened (nitro and nitroso products). The nitroso product itself strongly quenched O-1(2) phosphorescence, while (photo)decomposition of the nitroso group, presumably to nitric oxide (NO.), produced nitrite as a minor product. We propose that O-1(2) adds to the >C=N(O) bond in DMPO, producing a biradical, >C(OO.)-N-.(O). This biradical may follow one of two pathways : (i) It may be protonated and rearrange to a strongly oxidizing nitronium-like moiety, which could be reduced to the DMPO hydroperoxide radical DMPO/(O2H)-O-. while oxidizing another DMPO moiety to ultimately form DMPOX. The DMPO/ (O2H)-O-. could undergo further redox decomposition, e.g. via the known Fenton-like reaction, to produce both free (OH)-O-. radical and the DMPO/(OH)-O-. radical. (ii) The biradical >C(OO.)-N-.(O) may cyclize to a 1,2,3-trioxide (ozonide), which could open the pyrroline ring to form 4-methyl-4-nitropentan-1-al and 4-methyl-4-nitrosopentanoic acid. Because the oxidation of DMPO by O-1(2) leads to both rapid O-2 depletion and the formation of transients and products that might interfere with trapping and identification of free radicals, DMPO should be used with caution in systems where O-1(2) is produced.