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Summary. The kinetics of the complex formation reactions between monofunctional palladium(II) complex, [Pd( dien)Cl] +, where dien is diethylene triamine or 1,5-diamino-3-azapentane, with L-cysteine and glutathione were studied in an aqueous 0.10? M perchloric acid medium by using variable stopped-flow spectrophotometry. Second-order rate constants, {k_2}^{298}, were (3.89±0.02) 10 2?M -1s -1 for L-cysteine and (1.44±0.01) 10 3?M -1s -1 for glutathione. The negative entropies of activation support a strong contribution from bond formation in the transition state of the process. The hydrolysis of Pd II complex gave the monohydroxo species, [Pd( dien)(OH)] + and the dimer with a single hydroxo-bridge species, [Pd 2( dien) 2OH] 3+. L-Cysteine and glutathione ligands form complexes of 1:1 stoichiometry and a dimer with a single ligand bridge. The formation constants of the complexes were determined, and their concentration distribution as a function of pH was evaluated.