The influence of complexation by ethylenediaminetetraacetic acid (EDTA) on the toxicity of Cd(II), Cu(II), Cd(II), and Ni(II) was investigated. Result of the Microtox test, which is based on measuring the decrease of light emitted by Vibrio fischeri bacterium when exposed to a toxicant, was used as an indication of toxicity. The effect of pH and EDTA molar ratio that might potentially modify the percentage of chemical species in solution on toxicity was evaluated. In general, results indicate that toxicity decreases when increasing the pH value as well as the EDTA molar ratio. Chemical modeling was used to predict metal speciation and correlation analysis to relate chemical species with the obtained toxicity results. The species that most contribute to toxicity resulted to be MeCl(+) (Me = metal), which is formed as a consequence of the presence of the bioassay medium (2 % NaCl). A model that predicts metal-solution toxicity by using the chemical species, which most contribute to toxicity is proposed as a useful tool for toxicity assessment in waters containing metal ions and EDTA.