The proton ligand stability constants and stability constants of simple and mixed ligand complexes of zinc with pyridoxine, nicotinic acid, tetracycline, gentamycin, benzyl penicillin, ampicillin, streptomycin and ranitidine have been studied potentiometrically at 25±0.5°C and μ=0.1M(KNO3) in aqueous medium. Zinc forms 1:1 and 1:2 complexes with all the chelating agents except for nicotinic acid-ranitidine, nicotinic acid-ampicillin and nicotinic acid-gentamycin 1:2 complex species does not exist in the solution. The mixed ligand chelates are shown to have formed in simultaneous equilibria. The higher values of stability constants of mixed ligand chelates than the sum of individual log stability constants of the first and second ligand are discussed.