In this paper a heterogeneous lumped parameter model is proposed to describe the mass transfer of effective alkali during the kraft pulping of wood. This model, based on the spatial mean of the concentration profile of effective alkali along the chip thickness, enables the estimation of the effective diffusion coefficient that characterizes the internal resistance to mass transfer and the contribution of the external resistance to mass transfer which has often been neglected. The experimental data that support this study comprise measurements of alkali concentration in the liquid phases, both inside and outside the chips. The kraft pulping of Eucalyptus globulus was carried out using broad range of temperature (80-165 degrees C) and effective alkali charge (10-45 g Na2O (100 g odw)(-1)). The model describes the effective alkali concentration profiles in the free liquor along the cooking with accuracy. The effective diffusion coefficient has been expressed by an Arrhenius-type equation and the estimated value of the activation energy for diffusion (E-a=20.7 kJ mol(-1)) is in agreement with previous studies. The contribution of the external resistance to the overall mass transfer resistance showed to be significant and to increase with both temperature and effective alkali charge at industrial operation conditions. This result confirms the relevance of building a model that takes into account the heterogeneous nature of the kraft pulping of E. globulus. (C) 2007 Elsevier B.V. All rights reserved.