p-tert-Octylphenoxyacetic acid (t0P-OCH2COOH) was a better extractant for Cu(II) than was a commercial aliphatic carboxylic acid extractant, namely Versatic 10. In the present study, the synergistic effect of nitrogen-containing donors (N-donors) on the extraction of divalent base-metal ions using tOP-OCH2COOH was studied. Pyridine (Py), 2,2'-bipyridine (Bpy), and 1,10-phenanthroline (Phen) were added as synergists. The addition of N-donors to organic phases containing `0P-OCH2COOH enhanced the extractabilities of the divalent metals Mn(II), Co(II), Ni(II), Cu(II), and Zn(II). Addition of Phen was the most effective for the enhancement of metal extraction, but the selectivity was low. Synergistic extraction using a mixture of `0P-OCH2COOH and Py was more effective for the separation of divalent metal ions. A mixture of `0P-OCH2CO0H and Bpy gave moderate extraction and selectivity; the following reaction was suggested for the extraction of Cu(II) : C4) + Bpy(org) 1.5(t0P-OCH2COOH)2(0,e----, Cu2 (Bpy)(t0P-OCH2COOH)(tOP-OCH2C00-1,2(org) 2Eltao. C) 2014 Elsevier B.V. All rights reserved.