The thermodynamic study on the extraction of Fe(III) with 2-ethyl hexyl phosphonic acid mono 2-ethyl hexyl ester was carried out. In this method, the stoichiometry of extracted species of Fe(III) was determined by slope analysis method. The extraction proceeds with cation exchange mechanism and the extracted species is of the form FeR3.3HR. Further, temperature dependence of the extraction equilibrium constants was examined to estimate the thermodynamic functions (Delta H, Delta G and Delta S) for extraction reaction.