Multivariate Curve Resolution by Alternating Least Squares (MCR-ALS) is applied to voltammetric data obtained in the study of competitive complexation of Zn(II) and Cd(II) by alpha- and beta-domains of metallothionein (MT). The application of MCR-ALS allows the estimation of both the voltammograms and the concentration profiles associated with each electrochemical process. The complexity of the voltammograms obtained in titrations of the alpha- or beta-domains of MT with Cd(II) and later Zn(II) (or vice versa) prevents their direct interpretation using traditional electrochemical methods. But when MCR-ALS is applied, voltammograms can be interpreted rather satisfactorily in both qualitative and quantitative terms. MCR-ALS showed the formation of Cd2Znbeta dom and Cd3Znalpha dom complexes when both metals were competitively added. A method based on the combined use of voltammetry and some chemometric techniques is proposed. It can be useful for the metal speciation of environmentally relevant natural ligands.