One of foremost air-polluting which can be determined as being deterioration in the quality of the air is the emission of sulfur-dioxide gas. In this study, CaCO3 solution which is one of the method used for the removal of the waste gas SO2 from air was used. The neutralization of CaCO3 with H2SO4 was realized experimentally at the pH value that dissolution rate is maximum. The neutralization of limestone with H2SO4 was realized in a stirred continuous reactor. pH value of the medium was controlled by utilizing self-tuning PID (STPID) algorithm and the on-line computer control system. ARMAX was used as the system model. A pseudo-random binary sequence (PRBS) was utilized as a forcing function in order to identify the dynamics of the process to be controlled and the system output was measured. The model parameters were evaluated by using Bierman algorithm. The tuning parameters of the STPID controller were determined. (c) 2005 Elsevier B.V. All rights reserved.