The acidity constants of aspartic acid (ASP) were determined by potentiometric pH titration. The stability constants of the 1 : 1 complexes formed between M2+: Mn2+, Co2+, Cu2+ or Zn2+ and Asp2-, were determined by potentiometric pH titration in aqueous solution (I = 0.1 M, NaNO3, 25°C). The order of the stability constants was reported. It is shown that the stability of the binary M (Asp) complexes is determined solely by the basicity of the carboxylate or amin groups. The observed stability order for aspartate follows the Irving-Williams sequence. It is shown that Asp can exert a direct influence on reaction rate through both the kind of metal ions and the kind of donor groups of Asp.