Different materials transport mechanisms lead to distinctly different morphological evolution during the sintering of ceramic particles. These behaviors are analyzed using meso-scale, finite-element models based on rigorous formulations of coupled equations for continuum transport and interfacial phenomena. While such two-particle models are simplistic with respect to a real powder compact, they nevertheless provide important mechanistic understanding of the sintering behavior of different systems. Calculations clearly show how viscous flow mechanisms for glassy particles produce simultaneous shrinkage and neck growth due to the global nature of materials transport. In contrast, results for crystalline systems without grain boundaries show that the more localized nature of diffusive transport leads to neck growth with very little densification until late stages of sintering. Surprisingly, surface vacancy diffusion leads to system elongation before densification occurs. Changes caused by the presence of a grain boundary are discussed in a companion paper [Djohari, H., Derby, JJ., 2009. Transport mechanisms and densification during sintering: II. Grain boundaries. Chem. Eng. Sci., in press, doi:10.1016/j.ces.2009.05.022]. (C) 2009 Elsevier Ltd. All rights reserved.