The complexes formed in the cadmium(II) succinate (suc2-) hydrogen ion and in the cadmium(II) malate (mal2-) hydrogen ion systems in aqueous solution at 37 °C and I = 150 mmol·dm-3 (NaCl) have been characterized by means of glass-electrode potentiometry. The succinate and malate protonation constants were found to be 5.187 ± 0.001, 9.135 ± 0.002 and 4.618 ± 0.001, 7.804 ± 0.002, respectively. The formation constants for the complexes Cdsuc, CdsucH+, and Cdmal were found to be 1.63 ± 0.02, 6.26 ± 0.06, and 1.25 ± 0.05, respectively. Particular attention has been paid to the evaluation of the effect of possible systematic errors on the constant values determined. Reliable standard deviation estimations have been made by applying a Monte Carlo calculation technique. Cyclic voltammograms and differential pulse polarograms for both cadmium(II) systems were consistent with formation of weak, labile complexes.