Fluorescent potential-sensitive dyes (PSDs) were screened for use in potassium-sensitive optical sensor membranes. A potassium-sensitive plasticized PVC layer containing valinomycin as an ion carrier, lipophilic borate salt as an anionic additive, and a PSD was chosen as a model for evaluating the applicability of PSDs. A commercial carbocyanine dye (3,3′-dihexadecyloxacarbocyanine perchlorate) was found to exhibit the best properties in terms of signal changes, photostability and operational life-time. The sensor membrane responds reversibly to potassium ion, with fluorescence intensity changes exceeding 50% and response times being of the order of 1 min. The response to potassium is slightly pH dependent. Typically, an 8% change in intensity is observed over the range pH 5–8. The new sensor membrane exhibits significantly improved signal changes compared to previous optodes based on related sensing schemes. We also report the effects of different plasticizers and lipophilic additives on the response of the layers. The response mechanism is discussed with respect to morphology of the membrane.