A theoretical approach to describe the relationship between the interface properties and densification for sintering materials is presented. This approach is developed by assuming simplified grain boundary geometry, where scaling is imposed. In general, the results are in good agreement with theoretical calculations from other studies, as well as with available experimental data. The model predicts a higher densification rate in particle agglomerates, if grain growth is inhibited. Hence, qualitative predictions lead to a plausible explanation, for instance, for the effect of niobium doping on PZT ceramics experimentally reported in the literature. The bulk and shear viscosities, as well as the sintering stress are reported. (C) 2003 Kluwer Academic Publishers.