In this study, the reactivity of metal ions that exist in fuel oil with some ligands present either in sea water (inorganic ligands) or in fuel oil (mercaptans and organic acids) were compared based on their formation constants. The values of the formation constants of the metals with the main ligands in water are known. The complexation of the metals Ni(II), V(IV), Fe(II), Cd(II) and Pb(II) with two ligands, hexanoic acid and 1-propanethiol, was studied. These two ligands were chosen to be representative of carboxylic acids and mercaptans present in fuel oil. This study was performed in solution using potentiometry, UV-vis spectrometry and electrothermal atomization atomic absorption. The study of the distribution of species as a function of pH indicated that at approximately pH 8 the most stable species among all metals were those with the oil ligands. An oil spill simulation was also performed and the concentration of the metals was measured in function of time. Theoretical and simulation data alike have shown that the metal ions are retained in fuel oil, even when this oil is spread in the sea. (C) 2006 Elsevier Ltd. All rights reserved.