This study reports the effect of a nonionic perfluorinated surfactant, N-polyoxyethylene- N-propyl perfluorooctane sulfonamide (PFOSA), as additive of background electrolyte on capillary electrophoresis (CE) of common inorganic cations. The association constants ( K ass) for PFOSA estimated from the electrophoretic mobility of analyte cations were the order of Mg 2+ > Ca 2+ > Sr 2+ > K + ≈ NH 4+ > Na + ≈ Li +. The K ass values were larger than those for zwitterionic and nonionic surfactants with hydrocarbon moiety. Use of PFOSA made another essential contribution to the determination of inorganic cations in a protein-containing sample. This was considered because high solubility of PFOSA for proteins functioned as suppressor for protein adsorption to the capillary wall. Four inorganic cations, Na +, K +, Mg 2+, and Ca 2+, in human saliva sample were successfully determined by sample injection without any pretreatments except for filtration and dilution.