The influence of a preceding electrode process in the analytical use of a.c., differential pulse and other polarographic methods is discussed. Coupled chemical reactions, such as found with the Cr(III)/Fe(III) system, cause considerable interference in all the techniques examined, interference is readily recognized by a shift in peak potential and a change in the wave shape on addition of increasing concentrations of the more positively reduced species. With other systems, a depolarizer more negatively reduced than another can be determined over a wide range of conditions with three-electrode instrumentation. IR-drop problems with two-electrode systems place considerable restrictions on their use. With Zn/Cu and Cd/Cu and other systems, reliable determinations were possible only when the concentration of the more positively reduced species was less than 10-2 M.