The extraction of Eu3+ from an aqueous perchlorate medium of constant ionic strength (0.1 M NaClO4) was studied at different temperatures by 8-quinolinol (HQ) in benzene, carbon tetrachloride, and chloroform, and by a mixture of 8-quinolinol and 1,10-phenanthroline (Phen), allowing for elucidation of the thermodynamic parameters and the mechanism of extraction of the species formed. The results indicated that the extraction is enhanced by adduct formation in the organic phase, and that the main extracted species were EuQ(3). 2HQ, EuQ(3). 2HQ . Phen, and EuQ(3). 2HQ . 2Phen.