The thermodynamic model and experiments were used to study the peroxide-oxide bleaching solution. The solution properties such as pH have been measured and modeled both in a pure aqueous solution and with wood fibers. Further, the thermodynamic multicomponent model was used with kinetic constraints for pH calculations in a reactive solution. The knowledge of the pulp and solution properties along with the results of the model calculations could be used for optimization of the bleaching process with respect to reaction time and temperature.