The synthesis and characterization of two anionic water-soluble thiaporphyrins 5,10,15,20-tetrakis(4-sulfonatophenyl)-21-thiaporphyrin (Htsptp^4–) and 5,10,15,20–tetrakis(4-sulfonatophenyl)-21,23-dithiaporphyrin (tspdtp^4–) and derivatives of the former with Cu²⁺ and Ni²⁺{[Cu(tsptp)Cl]^4– and [Ni(tsptp)Cl]^4–} have been carried out. Both Htsptp^4– and tspdtp^4– show aggregation in aqueous solution. Addition of cation and 18-crown-6 (1,4,7,10,13,16-hexaoxacyclooctadecane) to dilute monomeric solutions of either results in cofacial dimer formation similar to that observed for normal anionic water-soluble porphyrins. The electronic absorption spectra of both the paramagnetic metal derivatives of the monothiaporphyrin show a split Soret band and a complex pattern of Q bands due to reduced symmetry. The fluorescence of the two thiaporphyrins is quenched considerably relative to H₂tspp^4–[5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin], the magnitude of quenching depending on the number of sulfur atoms in the porphyrin core. Electrochemical studies indicate harder oxidations and easier reductions for Htsptp^4– and tspdtp^4– relative to H₂tspp^4–. Addition of a first electron to [Cu(tsptp)Cl]^4– or [Ni(tsptp)Cl]^4– results in the reduction of the metal centre suggesting that the metal d_x²–y² orbital is lower in energy than the empty porphyrin e_g(π*) orbitals in contrast to those of [Cu(tspp)]^4– and [Ni(tspp)]^4–.