Abstract: The PVC membrane selective electrode for benzalkonium bromide was studied. It is based on the use of benzalkonium tetraphenylborate ion pair complex as the active material. The PVC membrane electrode showed Nernstian response over the concentration range from 10^-3 mol/L to 5×10^-6mol/L with a slope of 53～57 mV/pC, and the detection limit was found to be 3×10^-6 mol/L. The electrode showed rapid response and good reproducibility. The selectivity coefficients of 16 substances were determined. Using direct potentiometric determination, the recovery of pure benzalkonium bromide was 101.8%. The present method is more rapid and simpler than the volumetric method given in the Chinese Pharmacopoeia.