Abstract: Novel 3,7-disubstituted phenothiazinium salts are active in the photochemical disinfection of aqueous effluents. The novel salts are of the general formula ##STR1## wherein: either R.sub.1 is a saccharide residue;R.sub.2 is alkyl, cycloalkyl, aryl, aralkyl or heterocyclyl, any alkyl or alkylene chain being optionally interrupted by one or more heteroatoms, and Y is alkylene optionally substituted by alkyl, cycloalkyl, aryl or aralkyl; orR.sub.1 and R.sub.2 are each methyl and Y is a bond; and when R.sub.1 is a saccharide residue X is an anion selected from halogen, R.sub.3 CO.sub.2.sup.-, R.sub.3 SO.sub.3.sup.- and R.sub.3 OSO.sub.3.sup.-, wherein R.sub.3 is alkyl, cycloalkyl, aryl, aralkyl or heterocyclyl, andwhen R.sub.1 and R.sub.2 are each methyl, X is an anion selected from R.sub.3 SO.sub.3.sup.-, R.sub.3 OSO.sub.3.sup.- wherein R.sub.3 is as defined above and R.sub.4 CO.sub.2.sup.-, wherein R.sub.4 is an aldose or ketose residue, an N-protected .alpha.-amino acid residue or a .omega.-carboxy-.alpha.