The use of square wave voltammetry (SWV) at a hanging mercury drop electrode to study a chemical reaction preceding a reversible electron transfer (CE mechanism) is reported. Two cases were analysed : cadmium(II) + nitrilotriacetic acid and the cadmium(II) + aspartic acid systems. In both situations the homogeneous reaction in buffered solutions and in the presence of an excess of ligand was considered to be first order. Experimental data were analysed in terms of theory of SWV for kinetic systems. Rate constants of dissociation of the complexes of cadmium with nitrilotriacetic acid and aspartic acid were determined using a numerical procedure based on a one variable fit. The results obtained are in good agreement with published values where other techniques have been used, in particular cyclic voltammetry.