The changes in the coal macromolecular structure and devolatilisation during carbonisation were studied in relation to changes in molecular structure. The mechanism of the initial solvent swelling process of high rank coals changes from Fickian transport (controlled by diffusion) to Case-II transport (controlled by molecular structural relaxation) when the cross-links in coal macromolecular structure start to cleave by thermal decomposition. The kinetic relaxation constant for Case-II transport changes considerably with temperature and shows a maximum in the thermoplastic temperature range. The molecular structural changes during the carbonisation process were monitored by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. CHAr/CHAl. (the ratio of the relative intensity of aromatic C-H stretching modes to that of aliphatic C-H stretching modes) increases, while the relative intensity of methylene groups (-CH2) decreases with increasing temperature in the thermoplastic temperature. This is attributed to (1) the loss of aliphatic rich low molecular material, (2) the loss of aliphatic side chains attached to aromatic rings, and (3) the cleavage of methylene bridge as a result of thermal decomposition, which leads to the growth of the aromatic lamellae in the carbon structure and an increase in cross-link density of the macromolecular structure.