In this work, the complexation of cadmium and zinc ions by 3-[N-tris(hydroxymethyl)methylamine]-2-hydroxypropanesulfonic acid (TAPSO), a commercial biological buffer, was evaluated using three electrochemical techniques, at fixed total-ligand and total-metal concentration ratio and varied pH, at 25.0±0.1°C and ionic strength set to 0.1M KNO 3 . For both metal–ligand systems, complexation was evidenced in the pH range close to deprotonation of the ligand and the final models were optimised after a meticulous graphical analysis.For Cd–(TAPSO) x –(OH) y system, two complexes, CdL and CdL 2 , were identified in the buffering region of the ligand. The proposed final model for this system is: CdL, CdL 2 and CdL 2 (OH) with stability constants, as logβ, of 2.2, 4.2 and 8.6, respectively.For Zn–(TAPSO) x –(OH) y system, the complex ZnL is the main species formed in the buffering pH range. The proposed final model is ZnL, ZnL(OH) and ZnL(OH) 2 with overall refined stability constants (as logβ) to be: 2.5, 7.2 and 13.2, respectively.