Ion transfer reactions at single microhole interfaces were studied using only pure water as the aqueous phase electrolyte resulting in a significantly increased potential window. The organic phase supporting electrolyte concentrations were reduced to micromolar levels and the results obtained were analysed using the general theoretical relationships derived by Oldham (J. Electroanal. Chem., 250 (1988) 1) for steady state voltammetry with little added electrolyte. The voltammetric response of the single microhole interface was investigated with regard to size and shape and as a result a novel amperometric sensing system for ammonium ions is proposed.