Alkali fusion is applied to substitute a sulfonic acid group with a hydroxyl group in a substituted aromatic ring. Beside the main reaction, also side reactions appear, because the substituents in the aromatic ring are eliminated and the aromatic rings are polymerized. A kinetic model for the main and the side reactions in alkali fusion of a substituted aminobenzenesulfonic acid was developed. The substitution reaction of sulfonic acid with a hydroxyl group, the elimination of sulfonic acid and amino groups, the dealkylation reactions, and the polymerization reactions were included in the kinetic model. The system consisted of the main reaction and of 27 side reactions. The number of adjustable constants in the kinetic model was reduced by assuming that the reactivity of any functional group in the aromatic ring is not affected by the other substituents. The reactions could then be described with eight kinetic parameters only. The kinetic data were obtained from isothermal experiments carried out in a batchwise operating laboratory reactor at-the temperature range of 220-270 degrees C. The kinetic parameters were estimated from the experimental data using nonlinear regression analysis. The model agreed well with the experimental data, which implies that the proposed reaction scheme is suitable for the description of alkali fusion and that the assumption concerning the reactivities of the functional groups is reasonable.