Stability constants and extraction constants of nickel (II) and zinc (II) chelates of N-(p-substituted phenyl)-β-mercaptocinnamamides were determined by solvent extraction method in chloroform system. Stability constants of the nickel (II) chelates were smaller than those of zinc (II) chelates. The extent of the back donation from metal to sulfur is considered to be an important factor of the order of stability in nickel (II) and zinc (II) chelates. The contribution of adduct formation of nickel (II) chelate with pyridine to synergistic effect was indicated by solvent extraction and spectrophotometric method. Hammett substituent constants of the p-substituted groups were found to be correlated with the stability constants, adduct formation constants, frequencies of d-d bands or the chemical shifts of thiol proton, in the ligands, nickel (II) chelates and their adducts.