In order to expand the application of voltammetry to ion transfer at the micro aqueous \ organic (W \ O) solution interface, factors essential for the measurement of voltammograms were investigated systematically by cyclic voltammetry. The effects of the dimension of the microhole, concentrations of supporting electrolytes in W and/or O and kinds of O on the characteristics of the voltammograms were investigated using micro interfaces formed in microholes made through thin polyester films set between O and W. When chloroform was used as O and the ratio of the radius to depth of the microhole was large, a well defined voltammogram for the transfer of an ion such as the tetraethylammonium ion from W to O could be observed even when the supporting electrolytes in W were as dilute as 5 x 10(-5) M. Organic solvents of low relative dielectric constant, epsilon, and alcohols were found to be applicable to voltammetry at the micro W \ O interface as O in addition to those used in voltammetry with a conventional interface. Formal potentials for transfers of various ions at interfaces between W and O of low epsilon and alcohols were determined.