In this study, an adaptive pH controller is proposed to control the pH process efficiently. Dissociation constant and total ion concentration of a fictitious weak acid are estimated to approximate the equilibrium characteristic of the pH process by the on-line recursive least-squares method and the pH value is controlled by a nonlinear proportional-integral (PI) controller using the estimated process parameters. Simulation results show that the proposed control strategy promises to handle both the nonlinearities and time-varying characteristics of the pH process.