Abstract A recently introduced model describing the response curves of polymer membrane ion-selective electrodes (ISEs) is used here to optimize a Cd 2+-ISE with a view to achieve very low detection limits. In a first step, the selectivity behavior is determined for two membranes based on the ionophore, N, N, N′, N′-tetradodecyl-3,6-dioxaoctanedithioamide (ETH 5435), but having different concentrations of the ionophore and ion-exchanger. Based on these data, the optimal response curves are calculated with a model that takes into account the ion fluxes induced by interfering ions. The experimental results with different ionic backgrounds correspond very well with the predicted effects. The best lower detection limit of 10 −10 M or 11 ppt Cd 2+ is achieved at pH 7 with an ionic background of 10 −4 M NaNO 3.