Calcium ion-selective electrodes based on PVC matrices with membranes composed of calcium bis[di(p-1,1,3,3-tetramethylbutylphenyl)phosphate] sensor and trialkyl phosphate solvent mediator are described and compared with electrodes of the same sensor with di-n-octyl phenylphosphonate mediator. Trialkyl phosphates of sufficiently high viscosity, for example, tri-n-butyl phosphate, tri-n-amyl phosphate, tri(1,1,3,3-tetramethylbutyl) phosphate and tri-n-octyl phosphate make good solvent mediators and maintain the calcium ion selectivity of the sensor for the range of alkali metal and bivalent ions studied, namely, Na+, K+, Mg2+, Sr2+, Ba2+, Mn2+, Cu2+, Ni2+ and Zn2+. The trialkyl phosphate mediators provide commercially available alternatives to di-n-octyl phenylphosphonate, the best in the range examined being tri-n-amyl phosphate.