A study was conducted on the stoichiometry of the complex formed by the extraction of aluminium ions by didodecyl naphthalene sulphonic acid (HDDNSA) from phosphoric acid solutions. The experimental results were analyzed numerically by non-linear regression. It was found that aluminium is extracted in the form of the complex ion AI(DDNSA)(3). The value of the equilibrium constant of the reaction was also calculated. (C) 1997 Elsevier Science B.V.