In order to predict the distribution of chemical species of copper and cadmium in water, conditional stability constant and complexation capacity between copper or cadmium and natural aquatic sediment have been determined in a shallow lake in Haman, Kyungnam. Kinetic parameters were calculated by Langmuir isotherm equation. Conditional stability constant was log $K_{cuSed}=4.78 and log K_{cdSed}=4.45$. Complexation capacity was $1.70{ imes}10^{-4}$moles/g for copper and $5.54{ imes}10^{-5}$moles/g for cadmium. Accuracy of experimental values of conditional stability constant was checked by comparing the calculated concentration of the metals with the measured one. Relatively good agreement between these values was obtained. Relative errors were 8.9% for copper and 6.5% for cadmium. Data of the measured conditional stability constant were put into data base of MINEQL computer program, and concentration of various chemical species of copper and cadmium in a model aquatic system was calculated. Aquatic sediment was associated with copper at the concentration of $10^{-5M}(0.059g/ell)$10-5M(0.059g/l) and with cadmium at the concentration of $10^{-6M}(0.018g/ell)$, and it significantly influenced on the distribution of chemical species of the metals. This result showed that prediction of chemical species of the heavy metals in an aquatic system should be taken into account the influence of the sediment.