Sensors of calcium bis[di(4-alkylphenyl)phosphates](alkyl = hexyl, octyl and 1,1,3,3-tetramethylbutyl) in conjunction with dioctyl phenylphosphonate solvent mediator in PVC matrices give good calcium ion-selective electrodes with near-Nernstian slopes and detection limits of 1.9 × 10–6 to 2.7 × 10–6 M calcium ions in the absence of ion buffers. Electrodes with the octyl-based sensor, when calibrated at 25 °C with calcium-ion buffers, gave calcium-ion detection limits of 10–8 M. They also responded well at 50 °C. The e.m.f. response of the electrodes in 10–3 M calcium chloride in 10–1 M sodium chloride was stable within the pH range from below 5 to about 9. All of the above electrodes showed good selectivity towards calcium ions over sodium, potassium and magnesium ions. A range of di-n-alkyl phenylphosphonate solvent mediators (alkyl = pentyl, hexyl, heptyl, octyl, nonyl, decyl or undecyl) can be used with calcium bis[di(4-octylphenyl)phosphate] sensor but electrodes from the pentyl and undecyl compounds degrade slightly more quickly than the others. Decan-1-ol solvent mediator leads to characteristic interference from magnesium ions. Electrodes with membranes containing the neutral carrier calcium-ion sensor NN′-di[(11-ethoxycarbonyl)undecyl]-NN′-4,5-tetramethyl-3,6-dioxa-octane diamide with 2-nitrophenyl octyl ether as solvent mediator were capable of near-Nernstian calibrations and calcium-ion detection limits of less than 10–6 M but quickly deteriorated.