The theory of the transient potential response of ion-selective electrodes is presented. The mechanism of the potential response is considered using a model of the phase-boundary potential difference, and the transient potential response is considered in terms of the electrochemical principle of ion transfer, with a single ion transfer being assumed (i.e. no interference) across the boundary interface and the electric double-layer structure of the interface. The theory predicts the effect of the ion concentrations, the ion transfer kinetics and the differential capacity of the interface on the response time of ion-selective electrodes.