Abstract: Some primary acid-base reactions and pH-controlling processes including homogeneous and hetorogeneous chemistry and hydrogen buffer mechanism in natural waters are briefly described in this paper. Then how to use a computerized chemical equilibrium model to estimate and predict buffer capacity and susceptibility to acid precipitation of water systems are discussed. The paper recommends REDEQL-2 Chemical Equilibrium Program to being applied to acidified water on estimating buffer capacity. Therefor, the function and procedure in this scope are describe.