This paper presents a computer simulation to invest ig ate the speciation of aluminum in acidic natural waters equilibria with mineral kaolinite based on the chemical equilibrium calculation. Various factors affect ing the dist ribution and concent rations of aluminum complexes are discussed. Compared with the case of equilibria with gibbsite, some unique characterist ics are discovered. This simulation has been verified by analyzing practical water samples and found in near-perfect ag reement with the experimental result s.The uncertainties in thermodynamic constant s are evaluated by the Monte Carlo simulation.