This paper presents a concise computer simulation which utilizes thermodynamic relationships coupled with mass balance and electroneutrality constraints to investigate the buffering function of aluminum in acidic natural waters euqilibria with mineral kaolinite-Various factors affecting theoretical prediction of the pH values are discussed, such as the concentration of dissolved silica, concentration of dissolved inorganic carbon and temperature-The model is verified by analyzing practical water samples-The pH calculated is in good agreement with the experimental results-The uncertainties in thermodynamic constants and measured input data are evaluted by the Monte Carlo simulation.