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The influence of sulfonation degree (n) and central atom nature on singlet oxygen quantum yield (ΦΔ) were studied for a series of sulfonated phthalocyanine metal complexes MPcS n mix . It was found that in DMSO, where the studied dyes exist as monomers, ΦΔ values are independent of number and position of the sulfogroups and are equal to 0.11 ± 0.02, 0.19 ± 0.03, 0.37 ± 0.05, 0.38 ± 0.05 and 0.68 ± 0.10 for the metal-free and Mg , Gd , Al and Zn complexes, correspondingly. However, in aqueous solutions, aggregation of the dyes determines their photochemical activity. Substitution in adjacent to macrocycle 3 and 6 benzene positions interferes with aggregation compared to 4 and 5 positions, presumably due to enhanced steric hindrances in the former case. In the ZnPcS n mix series (n varied from 2 to 4), as an example, the linear relationship between degree of aggregation and ΦΔ indicates that only the monomer fraction of the dye is responsible for singlet oxygen production. For less sulfonated samples the order of activity of the dyes in water is as follows: H 2 PcS n mix < M II PcS n mix < M III PcS n mix which is in contrast to their tendencies to form aggregates.