The capacitance of various electrolytes in dichloroethane, anisole and cyclohexane solutions are measured at a platinum electrode. It is found that specific adsorption occurs at large overpotentials from the potential of zero charge (pzc), but a diffuse-layer minimum is observed close to the pzc. The diffuse layer capacitance is found to be dependent upon the ionic strength of the electrolyte solutions. The appearance of capacitance ‘humps’ as a function of potential is attributed to the adsorption of ions or ion aggregates. The desorption of ions is shown to be a very slow process, particularly when long chain quaternary ammonium cations are used and, hence, the double-layer capacitance appears to be time dependent. The slow desorption of ions leads to apparent artefacts when these electrolytes are used for voltammetric studies in non-polar solvents.