A commercially available polarographic instrument incorporates novel methodology designed to enhance the response of reversible analytes over that of interfering irreversible analytes, or vice versa. The principle involved is to sample the current flowing in response to a potential pulse at two different times. These two samples are then combined in two alternative ways : either to discriminate against an irreversible process (the DPR mode), or to discriminate against a reversible process (the DPI mode). The theory underlying these discriminatory modes has been developed and used to predict their efficiency. These predictions have been compared with experimental results for the reduction of the Cd2+, Zn2+ and Ni2+ ions at a mercury drop electrode, using a Metrohm instrument capable of implementing the DPR and DPI modes, in addition to the usual DPN (single current sample) mode of differential pulse polarography. There is qualitative agreement between theory and experiment, and both serve to validate the principle. The effectiveness of the DPR mode against the irreversible reductions of H- and dissolved 02 is also examined.