A methodology for the calculation of the equilibrium of the causticizing reaction of pure sodium carbonate solutions at 100-degrees-C without any adjustable parameters is presented. The methodology is extended to compute the effect of sodium sulfide. The causticizing reaction was found to be slightly endothermic at 100-degrees-C. The caculated results were found to be in good agreement with experimental data. An empirical kinetic model for the causticizing reaction is also presented. The model takes into account the variations in both green liquor composition and lime quality and can yield the causticizing efficiency directly. The calculated results of the causticizing efficiency were found to be in good agreement with experimental data from several Canadian pulp mills and reflected the effect of the variation in lime quality.