Abstract: The reaction rate between the gaseous chlorine and the lignin in plant cellulosic material under the conditions of the pulping process with chlorine is determined, and the course of the reaction discussed. The determination of the reaction rate is carried out by measuring at constant temperature the volume of chlorine gas absorbed by rice straw which has been pret-reated with dilute alkali. The results indicate that the course of the reaction may be analyzed into two parallel reactions: A rapid one, the rate of which resembles that of the second-order reaction with unequal initial concentrations, and a slow one, the rate of which resembles that of the second-order reaction with equal initial concentrations. The only difference is that the concen-trations applied in the general kinetic equations are replaced by the frac-tions of the reagents remained. Through the determination of the quantity of hydrochloric acid formed during the reaction it is possible to indicate that the rapid reaction is of substitution type and the slow one of oxidation. The relation between the reaction rate, the course of the reaction and the main conditions of the reaction such as the quantity of chlorine applied, the content of lignin in the cellulosic material and the reaction temperature may be represented by certain mathematical equations.