Plasticized PVC membranes incorporating the hexaester of calix[6]arene, potassium tetrakis(4-chlorophenyl) borate and a neutral H+-selective chromo-ionophore (ETH 5294) have been used as reversible sensing devices for optical determination of potassium ion in pH-buffer solutions. At pH 9.0, the absorbance response of the optode shows a good correlation with the theoretically derived formula in the range from 1 x 10(-6) to 1 x 10(-2) mol l(-1) potassium ion. Typical response times (t95) in the samples are 3 min, the relative standard deviation for repeated measurements being within 0.5%. The selectivity of this potassium-selective membrane allows its application for the reversible optical detection of potassium concentration in human serum samples.