Potassium ion-selective compositions which comprise valinomycin and a particular carrier solvent capable of solvating valinomycin are disclosed. The carrier solvent is a dicarboxylic acid diester having at least 25 carbon atoms, a viscosity of less than about 120 centipoise at 20.degree. C. and a boiling point greater than about 170.degree. C. at 5 mm pressure. A preferred carrier solvent is diisodecyl phthalate. Membranes prepared from these compositions preferably have a glass transition temperature greater than about -50.degree. C. These membranes are particularly useful in dry-operative potassium ion-selective electrodes which are capable of selectively determining potassium ions in preference to other ions.