In alkaline solution nitromethane (NM) forms a stable aci tautomer (H2C double bond NO2-) which may be used as a spin trap for radicals, including NO. and NO2.. We have noticed previously that in aerated photochemical systems NM can undergo photosensitized degradation (Bilski et al., J. Am. Chem. Soc. 1992, 114, 549), possibly via singlet oxygen (O-1(2)). We have now confirmed that aci-NM does indeed quench O-1(2) phosphorescence (k(q) 2.4 X 10(7) M(-1) s(-1)), and that the addition of O-1(2) to aci-NM is solely responsible for this chemical quenching, as confirmed by the high quantum yield of O-2 photoconsumption (phi = 0.65 for [NM] = 35 mM). To obtain "labeled" fragments from the decomposition of the resultant aci-NM-O-2 peroxy species we have applied another spin trap, 5,5-dimethyl-1-pyrroline N-oxide (DMPO), in a novel way : we used an EPR silent hydroxylamine adduct (DMPOH/CH double bond NO2-) formed by nucleophilic addition of aci-NM to DMPO. Reaction of DMPOH/CH double bond NO2- with O-1(2) resulted in the generation of the DMPO/CO2.- radical, suggesting that in the absence of DMPO the following mechanism may occur : O-1(2) + CH2 double bond NO2- --> NO2- + HCOOH. We have found that nitrite and formate production accounts for most of the oxygen consumed. This suggests that O-1(2) reacts mainly with the carbon atom of NM producing a biradical transient which decomposes yielding thermodynamically stable products nitrite and formate. A minor product is peroxynitrite (OONO-) which must be produced via an ozonide-type intermediate formed by intramolecular recombination of the biradical. Prolonged irradiation of Rose Bengal and aci-NM resulted in accumulation of nitrite which was then photooxidized to NO2., trapped by unreacted aci-NM and identified as the NM/NO2. spin adduct.