The solvent extraction of copper (II) from acetate buffers by solutions of acetylacetone (HX) in benzene is shown to be synergistically enhanced by the addition of the heterocyclic bases (B) quinoline or isoquinoline due to the formation of 1:1 adducts, CuX₂B. These adducts have also been prepared in the solid state and their structure is discussed. The formation constants K_2,1 = [CuX₂B]_org./[CuX₂]_org.[B]_org. are calculated to be 3·0 ± 0·7 and 5·2 ± mole⁻¹ l. from the distribution measurements and 3·7 ± 0·2 and 8·6 ± 0·4 respectively from direct spectrophotometry in dry benzene.