Solvent swelling experiments of coal particles of Taiheiyo and Witbank coals in pyridine at 15-50 degrees C were conducted by using an orthogonal microscope coupled with an image analysis system and the newly prepared temperature-controlled cell. Two kinetic parameters, n (a number that crudely indicates the nature of the solvent diffusion) and k (a rate constant), were calculated based on time profile of volumetric swelling ratio. The authors examined the correlation between these parameters and volumetric swelling ratios, to find that there was linear correlation between n and In(1/k). By using the proportional constant, the authors calculated activation energy of the swelling of both coals. The energy changed with respect to each particle. Average values were 71kJ/mol for Taiheiyo coal and 55kJ/mol for Witbank coal, these values agree well with the values reported in the literature. The authors also examined the effects of pretreatments of both coals. O-Acetylation and pyridine preextraction lead to decrease the energies drastically. (c) 2007 Elsevier B.V. All rights reserved.