Pa( V ) was extracted from perohloric-perchlorate of 1M ionic strength solution at different temperatures. Thermodynamic parameters of PaO( OH) [ H( DEHP)2]2formed In the organic phase were calculated and discussed. Some organophosphorus compounds : trloctyl phosphlne oxide ( TOPO) : triphenylphosphine oxide ( TPPO; and tributyl phosphate ( TBP) were used as adducts mixed with HDEHP for Pa( V ) extraction. The distribution ratio studies suggest the following species in the organic phase : PaO( OH) [H( DEHP )2]9.TOPOt PaO( OH) [ H( DEHP) ]2. TPPO and PaO( OH ) [ H( DEHP)2]2PaO( OH) [ H( DEHP) ]22TBP. The data obtained Tor δ H, δ G and δ S of the synergistic speoles were discussed and the reaction mechanisms were proposed. Antagonism was observed in the extraction by HDEHP-TBP at low concentrations of the TBP.