An electrochemical cell is described which allows measurements of ionic and electronic conductivities of mixed conductors as a function of composition. On one side of the disc-shaped sample four peripheral, roughly equidistant electronic contacts are mounted; on the other side four ionic contacts are positioned in the same manner. As the cell is based on the van der Pauw geometry, only the thickness of the sample and not the spacings between the electronic and ionic contacts needs to be determined. The composition of the sample can thus be altered by means of coulometric titration. For conductivity measurements on mixed conductors the composition gradient of the sample, arising from the flow of electrons and ions in the mixed conductor, is considered in detail. To demonstrate the high stoichiometric resolution the cell has been applied to study the composition dependence of ionic and electronic conductivities of alpha-Ag2Te at 300 degrees C within the homogeneity range and with high stoichiometric resolution. Experimental details as well as an interpretation of the composition dependence of the electronic conductivity are given.