Optimization of the reaction conditions in liquid phase semibatch reactors where several complex reactions simultaneously take place was studied. The optimization procedure was demonstrated with the production of substituted alkyl phenols from aromatic sulfonic acid via alkali fusion. The reagent, the substituted aromatic sulfonic acid, participates in the main reaction and in several side reactions, such as the elimination of the alkyl, the amino, and the sulfonic acid groups as well as polymerization reactions. The kinetics was determined for the reaction system, on the basis of the laboratory scale batch reactor experiments. The kinetic model was combined with a large scale reactor model, where not only the feed of the reactant but also the precipitation and volatilization of some reacting compounds were considered. It turned out that an extrapolation from the 0.1 dm 3 laboratory reactor to the 10 m 3 large scale was very successful. The reaction conditions of industrial scale semibatch reactor were optimized using the desirability function approach, which combines the desired conversion of the reagent, the desired selectivity of the product, and the reaction time. The qualities of different desirability functions were compared. The great benefit of the desirability approach is that it provides smooth objective functions, avoiding a cumbersome constrained optimization.