A reliable method based on electrochemical measurements for the evaluation of protein interaction with small electroactive coordination compounds is proposed. Protein binding capacity of five different cationic Re(V) complexes was evaluated by means of voltammetric techniques in the absence and the presence of bovine serum albumin. The percentage of interaction between the complex and the protein was estimated from the quantitative decrease of the diffusion coefficient for the sole complex due to the addition of the protein, and therefore the formation of the protein-complex molecular ensemble. The proposed methodology was checked using cisplatin as a molecular complex probe.