The influence of hydrogen, lithium, and cesium ions on the potential of calcium-selective electrodes containing thenoyltrifluoroacetone as a complex-forming ion exchanger in the membrane was investigated experimentally. The fit of the experimental data to Morf's equation was estimated quantitatively. It was shown that the 'anomalous' behavior of the calcium membranes is a normal phenomenon produced, not only by hydrogen ions but also by other ions strongly associating with the exchanger.