Wood impregnation is crucial in any pulping process. The impregnation of chips of eucalyptus wood by a liquor of constant alkali concentration is here analyzed. A mass balance that considers kinetics of the main reactions and a dynamic diffusion coefficient is applied for the six chemical species analyzed. The diffusion rate is assumed dependent on temperature, alkali concentration, and the advance degree of reactions. The predicted concentration profiles are in acceptable agreement with the experimental results for hidroxyl and sodium ions, as well as for acetyl groups. For each set of impregnation conditions and chip thickness, characteristic impregnation times or impregnation levels can be predicted by the model.