A filter function is a mode of pulse voltammetry that removes completely the current response for a reversible charge transfer reaction for all values of potential or gives a constant response (not equal 0) for all values of E. These modes will be designated as filter functions of the first and second class, respectively. With both kinds of functions it is a key condition to obtain the corresponding effect that the current is sampled at selected values of the pulse durations, i.e. if the times for the potential steps are chosen arbitrarily the filtering effect is not observed. In turn, and because for a slow charge transfer reaction the current response with a first class filter function is not zero (nor constant when a second class filter function is applied), we have that the use of these modes of pulse voltammetry provides a way of obtaining without interferences the signal for an irreversible electron transfer in those cases where reversible processes are also present. Thus, if reversible and irreversible electron transfers with close discharge potentials occur simultaneously the response observed when a first class filter function is applied shows only the contribution due to the irreversible process while with a second class mode this last response is superimposed to the constant output obtained for the reversible process. In this paper the theory for these kinds of filter functions is established, particularly for double and triple pulse potential step techniques, and the characteristics of the responses obtained with some of these modes is discussed. Finally, the characteristics of the response obtained with filter functions at spherical electrodes are also shown.