This study focuses on the thermodynamics and kinetics of the complexation of nickel(II) ions to aspartic acid (Asp), the building block of poly-aspartic acid (PAsp), a biodegradable polyelectrolyte that is extensively used for water and protein purification. Similar to other toxic heavy metal ions, the toxicity of Ni2+ is determined mainly by pH and its speciation. The studies have been performed in aqueous solution, at constant temperature (25°C) and ionic strength (0.1 mol l-1 NaCl), between pH 3 and 8 applying potentiometry, UV-Vis-spectrophotometry and stopped-flow technique. These conditions represent the ionic strength and the pH in natural environments (surface waters, soils, plants).