The oxidation rate of an aqueous solution of 8-amino-1-naphthol-3,6-disulfonic acid was determined at 140 ^oC and 4.5 MPa from the difference of overall pressure changes in an autoclave. Additionally, the reaction rate was evaluated from the on-line recorded values of temperature and pressure by means of a 16-bite ADC converter. The principles of connection, smoothing of signals, and physico-chemical factors affecting the accuracy of results are discussed. Special attention is paid to the influence of partial pressures of solvent, oxygen, and gaseous oxidation products. The time dependence of reaction rate is obtained by derivation the regression polynomial of the integral time dependence of reacted oxygen.