Subtractive polarography is a differential polarographic technique that allows the measurement of wanted signals in the presence of unwanted signals. The technique requires two polarographic cells, with DME's operated at the same potential. The “unknown” solutionis placed in one cell and gives rise to the total current. The solution in the second cell contains those components of the “unknown” solution that give rise to the unwanted portion of the total signal. Measurement of the difference between the polarographic currents at the electrodes in the two solutions thus yields the wanted portion of the total signal. This technique has been evaluated with a controlled-potential differential dc polarograph that utilizes dropping-mercury electrodes having uncontrolled drop times, and the results are presented in this paper. The results indicate that subtractive polarography affords improved sensitivity, selectivity, and resolution over that of conventional dc polarography, and an improvement in resolution over that of derivative polarography. An additional improvement in resolution can be achieved by means of derivative subtractive polarography.