High-temperature rheometry and H-1 NMR have been combined to assess the microstructural changes taking place during carbonization of a number of different coals. A linear relationship exists between the logarithm of the material's complex viscosity (eta*) and the fraction of hydrogen present in rigid structures (phi(rh)) for the resolidification region in which the material is liquid-like with small amounts of dispersed solid. The relationship is best characterized by the Arrhenius viscosity equation given by eta* = eta(0)* exp([eta]phi(rh)), where eta(0)* is the complex viscosity of the liquid medium and [eta] is the intrinsic viscosity of the resolidified material. Attempts to fit the Krieger-Dougherty suspension equation showed that the solid regions formed do not pack together like a normal suspension. Instead. it is more likely that cross-linking and cyclization reactions within the liquid medium give rise to a network structure of solid material and a characteristic gel point. The ratio of hydrogen present in rigid structures to that still present in liquid form at the gel point is approximately 2:3. The resolidified material was found to have a higher [eta] than the components of the coal that remained unsoftened, which suggests that while the unsoftened components have a fairly equant shape, the resolidified components have a much higher hydrodynamic volume. The resolidification process bears similarity with thermosetting polymer networks and the eta* measurements taken for a blend of two coals follow a common two-component polymer blending rule. (c) 2006 Elsevier Ltd. All rights reserved.