A novel type of optical potassium sensor is presented whose response mechanism is virtually insensitive to pH and ionic strength. It is based on a coextraction mechanism using a lipophilic ion carrier and a fluorescent anion, the two contained in a solid oil-in-water emulsion. The latter is composed of lipophilic plasticizer droplets (containing valinomycin) and a hydrogel containing the highly solvatochromic anionic dye merocyanine 540. On exposure to a solution of potassium ions, these are extracted, along with the anionic fluorophore, into the plasticizer phase wherein the dye is much more fluorescent than in hydrogel. The dynamic range of sensor response can be adjusted by variation of the membrane composition. Membranes were optimized for measurements in the physiological range. The dynamic range is from 0.1 to 50 mmol/L K+. Response times are <3 min within this concentration range.