The extraction constants (K*) for zinc(ll), cobalt(ll), and nickel(ll) were determined for the extraction of the metal ion from an aqueous phase into a chloroform solution of dithizone. The extraction constant for nickel (0.60) was significantly different from that reported by Koroleff (1.2 -3). This difference is attributable to the slow formation of the nickel dithizonate. As Zn(ll), Co(ll), and Ni(ll) cannot be separated by extraction with dithizone without the use of masking agents, the optimum conditions for the separation of the ion pairs Ni-Zn, Zn-Co, and Ni-Co, based on the differences in the rates of formation of the metal dithizonates, were determined. More than 99% of Zn could be extracted while only 2.5% of Ni extracted. The optimum conditions determined for the separation of Zn from Co resulted in extraction of 96% Zn with only 8.6% Co extracting. The separation of Co from Ni was less effective, because of the similarity in extrac-'To whom inquiries should be addressed.