A novel potentiometric membrane sensor for potassium ion based on the use of rifamycin as a neutral ionophore is described. The sensing membrane is formulated with 2 wt.% rifamycin-SV, 69 wt.% dibutylsebacate plasticizer and 29 wt.% PVC. Linear and stable potential response with near-Nernstian slope of 56.7 ± 0.2 mV decade −1 are obtained over the concentration range 1 × 10 −1−3 × 10 −5 M K +. The detection limit is 0.3 μg ml −1 K +, the response time is 10–30 s and the working pH range is 4–11. Responses of the sensor toward alkali and alkaline earth metal ions are in the order K + > Rb + > Cs + > Na + > NH 4 + > Ba 2+ > Mg 2+ > Ca 2+ > Sr 2+ > Li +. The selectivity coefficient data reveal negligible interference from transition metal ions. Direct potentiometric determination of K + in the presence of 10–50-fold excess of alkali and alkaline earth metals gives results with an average recovery of 99.1%, and a mean standard deviation of 1.2%. The data agree fairly well with those obtained by flame photometry.