Traditionally, chemical reaction between solids has been considered to typically occur on a geological time scale without the benefit of high temperature, due to diffusion block in the solids. However, recent advancements have revealed that many solvent-free reactions between molecular crystals can quickly occur at room or near-room temperature. These reactions have raised a novel scientific question as to how the reactive species can overcome the diffusion-controlled kinetic limitations under such moderate conditions. From time-resolved powder UV-vis reflection spectra and optical micrographs with the reaction between dimethylglyoxime and Ni(Ac)2 · 4H 2O and the reaction between hexamethylenetetramine and CoCl 2 · 6H2O as models, we found that the solvent-free reaction really occurs at an intermediate state between the solid state and the liquid state. Formation of the liquid phase provides a convenient approach to diffusion of reactive species, whereas formation of a solid product layer hampered the transfer of reactive species. Both factors led to a broad reactive rate band in the long reaction region. The results have explained the diffusion mechanism of the fast reaction between the molecular crystals under moderate conditions. © 2008 American Chemical Society.