In the present investigation, the stability constants of zinc pyrazolone complexes in various solvents and the synergistic extraction equilibrium constants of several synergistic extraction systems have been determined. The results are listed in table 4 to 8 in the Chinese Teat. It is found that the inflnenoe of inert solvents on the extraction equilibrium constants may be interpreted approximately in terms of theory of regular solutions, but the influence of noninert solvents containing donor atoms is more complex. For the synergistic extraction systems, the equilibrium instants vary greatly with different solvents. Among other factors, the change of the activity aoeffloient of the neutral ester with the solvent plays an important role.