The extraction of magnesium ions from phosphoric acid using dinonylnaphthalene sulfonic acid (DNNSA) was studied. The effects of the extraction time, diluent, phosphoric acid concentration, DNNSA concentration and temperature were examined. The H3PO4 concentration had a negative effect on magnesium ion extraction, while the DNNSA concentration had a significant positive effect. The DNNSA easily forms reverse micelles in the organic phase. The experimental results were analyzed mathematically by nonlinear regression to determine the stoichiometry of the complex formed in extraction. It was found that magnesium was extracted in the form of the complex with HA representing the free acid form of DNNSA. The equilibrium constant of the extraction reaction was calculated to be 59.6. (C) 2009 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.