Abstract: The extraction of UO2C12 from HC1-solution with 30%, 40% and 50% (V/V ) trioctyl amine-toluene is studied. The activity coefficients of HC1 and UO2C12 in aqueous phase are calculated by Pitzer's equation and improved Frank-Thompson's equation. The activity coefficients of each compound in organic phase are obtained as follows: water——by Karl Fischer titration method and Pitzer's equation; diluent——by head space method; extractant and extracted compounds——bythermodynamic method. From the data obtained, the extraction ther-modynamic equilibrium constants are calculated to be 1.46×103 and ( 2.48 -3.79)×105 for HC1 and UO2C12 respectively.