A new potassium ion-selective membrane electrode has been developed based on crown ether. The membrane consisted of benzo-15-5 crown ether as a neutral carrier, orto-nitrophenylphenyl ether as plasticizer and PVC as membrane matrix. An analytically useful potential change occurs in the 1x10(-6) - 1x10(-2) mol L-1 concentration range. The slope of the linear portion (1x10(-2) - 1x10(-4) mol L-1) was 56 +/- 3 mV/10-fold concentration changes in potassium concentration. In the presence of 1x10(-3) mol L-1 potassium, no interference was observed for Li+, Na+, NH4+, Mg2+, Ca2+, Sr2+, Ba2+, Cr3+, Mn2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, F-, Br-, I-, NO2-, SO42-, CO32-, S2- ions. The measurements were made at constant ionic strength (0.1 mol L-1 NaNO3, medium) and at room temperature. The lifetime of the electrode was approximately 2 months, and the response time was about 20-30 seconds. This electrode has been used for the determination of potassium ion with standard addition method in orange juice, vitamin, and edible salt. The amount of K+ found was 1.97 +/- 0.06 mg/mL in orange juice, 0.420 +/- 0.008% (m/m) in vitamin tablet and 0.52 +/- 0.08% (m/m) in edible salt.