A method is described for eliminating the undesirable effect of the double-layer capacity current on the sensitivity of a n A.C. polarograph. A square-wave voltage is used in place of the usual sinusoidal polarising voltage, and the amplitude of the A.C. component of the cell current is measured shortly before each sudden change in the applied voltage. Preliminary results suggest that the method is useful for the detection, at concentrations down to 2 x lo-' A:, of ions that are reversibly reduced,