Precise quantum yields for generation of singlet molecular oxygen, O-2((1) Delta(g)), have been measured for tetrakis(4-sulfonatophenyl)porphyrin (TSPP), closely-related water-soluble porphyrin derivatives, and tris(2,2’-bipyridyl)ruthenium(II) (bpy(3)Ru(2+)) in O-2-saturated water. Under the experimental conditions, TSPP is present in an aggregated state, but the measured quantum yield for formation of O-2((1) Delta(g)), Phi(Delta) = 0.51, remains on the same order as that of the corresponding monomer in methanol, Phi(Delta) = 0.70. Comparison with the quantum yield for formation of the triplet state indicates that quenching of the triplet by O-2 gives O-2((1) Delta(g)) With an efficiency of ca. 80%. There is a small but significant increase in Phi(Delta) when D2O is used in place of H2O. For the other porphyrin derivatives, the ability of the aggregate to sensitize formation of O-2((1) Delta(g)) appears to depend on the total electronic charge resident on the molecule, decreasing with decreasing negative charge. A strong solvent dependence noted for O-2((1) Delta(g)) production with (bpy(3)Ru(2+)) is explained in terms of competition between triplet energy transfer and photoinduced electron (or partial charge) transfer. Similar behavior could be responsible for the relatively low Phi(Delta) value observed for TSPP.