Hydrodynamic properties, such as solution viscosity and diffusivity, were measured at 298 K to investigate the solution state characteristics of coal extracts in pyridine obtained from Upper Freeport (UF) and Illinois No. 6 (IL) coals. The solvation constants obtained by applying the Pal-Rhodes equation to the viscosity of the extracts in pyridine are 2.9 and 4.6 for the UF and IL extracts, respectively. A larger content of acidic hydroxyl groups in IL extract than UF extract make pyridine solvate IL extract more extensively than UF extract. However, the solvation. constant of the O-methylated IL extracts is 9.3, unexpectedly larger than that of the original IL extract. To have further insight into this observation, the change in the molecular shape of the extracts by the O-methylation were examined, using the combination of intrinsic viscosity and diffusion coefficient. The shapes of the coal extracts have been found to be prolate ellipsoids with axial ratio of 6.5 and 4.0 for UF and IL extracts, respectively. The axial ratio of the O-methylated IL extract is 13.1, suggesting that the elimination of the hydrogen bonds or the steric effect of added methyl makes the shape of IL extract stretched out. This deformable, nature of coal extracts makes the interpretation of the solvation constants from Pal-Rhodes, equation inaccurate, as it assumes that the particles are monodispersed hard spheres.