In this study, a system consisting of Pb, Cu and Cd as typical heavy metal pollutants, and purified Aldrich humic acid as a representative of natural organic materials were selected as prototypes La model the environmental system. The effect of environmental factors such as pH, salinity, and concentration on the interaction between metals and humic acid were investigated in detail. The experimental results show that pH and ionic strength are: the most important variables in controlling metal adsorption on humic acid. The results also show a high complexation capacity of humic acid for the metals, especially Cu and Ph, The strength of binding of the three metals is in the sequence of Pb > Cu > Cd. A theoretical model featured by surface complexation reactions and double layer theory combined with the Poisson-Boltzmann equation was applied to simulate the experimental data. Titration data of humic acid with NaOH and Ba(OH)(2) were used as concentrations of strong and weak acid groups on HA and the input; data for the model calculations. Intrinsic adsorption constants and capacitance are estimated based on literature values and optimized to obtain the best agreement between the experimental and the model results. The model was also used to predict surface properties such as the speciation of the metals adsorbed on humic acid. Good agreement between theoretical modeling results and experimental data suggests the applicability of the theoretical model to this system. Given these parameters, theoretical calculations obtained using the model proposed in this study can give a detailed picture of the actual environmental conditions. Also, the model calculations will help in the evaluation of the actual toxicity of heavy metal pollutants in the aquasystem.