Cationic phthalocyanine and/or porphyins salts are used as photosterilising agents for microbiologically contaminated waters, particularly for inactivating Gram-positive and Gram-negative bacteria, yeasts, micoplasmas and parasites. Synergistic action for water photosterilisation by the simultaneous use of the cationic phthalocyanine salt and a porphyrin molecule, which performs its photosterilising action by a mechanism identical with that typical of phthalocyanines, and whose visible light absorption spectrum efficiently complements the absorption of phthalocyanines. The photosterilising salts can be added to the water as a solution, or as a powder or tablet.