The excited triplet states of a set of water-soluble 4-sulfonatophenyl meso-bis and meso-tetrakis substituted porphyrinoids were studied in a wide range of concentrations. All of these porphyrinoids show the formation of pi-stacked aggregates (H aggregates). As a consequence of the 5,15-diaryl Substitution, and of aggregation, two excited triplet states can be detected, the more stable being accessible only by internal conversion of the current excited-state T-1. The aggregates showed a significant singlet oxygen quantum yield [O-1(2) ((1)Delta(g)), determined by the phosphorescence method], which in most cases was of the same order as the monomeric forms.