Bromley＇s theory for calculating activity coefficients in order to correlate the values of cadmium extraction constant by Cyanex 302 from phosphoric acid solutions at different ionic strengths has been applied. A chemical model for the aqueous phase including the species H3PO4, H2PO4-, H5P2O8-, H6P2O8, CdRPO4 and CdH2PO4+ has been considered. The increase observed for the extraction constant value when increasing the phosphoric acid concentration is probably due to the significant increase of the cadmium activity coefficient. A reaction extraction including water as a component has been proposed, and the value of the thermodynamic extraction constant of log K-0 = 7.02 for the formation of CdR2(HR) species, HR being the major component of Cyanex 302, has been obtained.