Ion-pair complexes of sulphonamide derivatives with quaternary phosphonium and quaternary arsonium have been synthesized and their electrode performances studied. It has been found that the electrode functions depend mainly on the site cation rather than on the anionic species and deteriorate in the order of cetyhrioctylammonium>cetyltriphenylphosphonium> cetyhriphenylarsonium. The electrode selectivity for different sulpha-drugs decreases in the order of sulphathiazole>sulphamethoxazole>sulphadimethoxine, sulphadoxine>sulphacetamide>sulphadiazine>sulphanilamide. Selectivity coefficients depend not only on the molecular connectivity index χ, but also on the electronic charge at the ionizing amide nitrogen atom σ and pK, according to the following formula: logK_(ij)=aχ+bσ—CpK_a—d. The quaternary phosphonium type electrodes have been suggested for use in the potentiometric determinations of sulpha-drugs. Ion-pair complexes of sulphonamide derivatives with quaternary phosphonium and quaternary arsonium have been synthesized and their electrode performances have. The has been found that the electrode functions depend mainly on the site cation rather than on the the anionic species and deteriorate in the order of cetyhrioctylammonium> cetyltriphenylphosphonium> cetyhriphenylarsonium. The electrode selectivity for different sulpha-drugs decreases in the order of sulphathiazole> sulphamethoxazole> sulphadimethoxine, sulphadoxine> sulphacetamide> sulphadiazine> sulphanilamide. Selectivity factors not only on the molecular connectivity index χ, but also on the electronic charge at the ionizing amide nitrogen atom σ and pK, according to the following formula: logK_ (ij) = aχ + bσ-CpK_a-d. The quaternary phosphonium type electrodes have been suggested for use in the potentiometric determinations of sulpha-drugs.