This paper reports on the use of multiple square wave voltammetry, MSWV, for the determination of europium(III) preconcentrated by ion-exchange at glassy carbon electrodes coated with Nafion(R). For this aim, a double-differential (DD) mode of MSWV is implemented; with this mode, background signals are always straight lines while the analyte signal is given as a twin opposite peak system which corresponds approximately to the instrumental double differential of the classical voltammetric wave. The influence of scan parameters and experimental conditions on MSW-DD responses for europium(III) determinations at the modified electrode is examined. Detection limits and sensitivities obtained using MSW-DD are significantly superior to those achievable by classical pulsed techniques. The method is applied to the determination of ultratrace europium(III) concentrations in the 10(-11)-10(-10) hi range in the cooling water of a nuclear plant; results obtained by the MSW-DD ion-exchange voltammetric approach compare well with those obtained by ICP-MS.