We report on a modified charge-pulse relaxation technique applied at planar lipid membranes. The method has an improved time resolution of 20-30 ns. It is based on the capacitive coupling of a voltage-jump to the membrane. The method was used to study the fast relaxation processes induced by valinomycin/K+ in the presence of 2,4-dichlorophenoxyacetic acid (2,4-D). The change of the rate constants of the ion carrier valinomycin was analysed as a consequence of the adsorption of the dipolar substance 2,4-D to the membrane/water interface of monoolein membranes. The effect of 2,4-D can be explained solely via the influence of the introduced dipole potential, VD. The latter was found to act (primarily) on the inner membrane barrier experienced by the positively charged carrier-ion complex and on the interfacial barriers responsible for complex formation and dissociation. No evidence for a change of the microviscosity of the membrane interior was obtained.