The thermodynamics of extraction of some lanthanide ions and bismuth, aluminium, calcium and zinc ions from perchloric acid into solutions of 2-ethylhexylhydrogen-p-phenylphosphonate have been studied. The extractions have been compared with those that employ dinonylnaphthalenesulphonic acid and factors that are responsible for the greater selectivity of the phosphonate have been elucidated. Charge density of the cation has a major influence on the mechanism of the reaction and in turn upon the thermodynamic parameters.