A luminescence decay time-based potassium ion sensor is presented which applies the ion-exchange principle. The bulk membrane consists of valinomycin, plasticized PVC, and the ruthenium(II) tris-4,4‘-diphenyl-2,2‘-bipyridyl bromothymol blue ion pair [Ru(dibipy)3](BTB)2 as a proton donor. The efficacy of radiationless energy transfer from the donor (the ruthenium complex) to the acceptor (BTB) is mediated by the potassium ion concentration. The concentration of potassium ions can be calculated from either luminescence intensity or decay time. At pH 8.7, the working function ranges from 1 to 100 mM KCl.