During pyrolysis and carbonization of coal, the viscoelastic properties vary across a wide range, with complex viscosity (eta*) decreasing to as low as 100 Pa s before increasing to approximately 10(8) Pa s and phase angle (delta) varying from close to 90 degrees (Newtonian liquid) down to 0 degrees (Hookean solid). A new rheometry method has been developed that combines tests using 25 and 8 mm plates to enable measurements of the entire resolidification process. When combined with thermogravimetric analysis, the method has provided new insights into the mechanisms leading to high and low IRSID, I40 strength indices. Although coals with very different volatile matter contents have similar rates of volatile release above 475 degrees C, viscoelastic properties above this temperature are highly variable. From a study of 13 coals, all coals for which delta < 55 degrees at 475 degrees C had an I40 index < 44%, while all coals for which delta > 65 degrees at 475 degrees C had an I40 index > 44%. It is thought that, when delta < 55 degrees, the material is less able to deform and accommodate the loss of mass/volume, causing it to crack/fissure. However, when the delta > 65 degrees and stays high until higher temperatures, the material is able to contract as volatiles are released without fissuring, ultimately leading to a higher I40 index. A relationship between the final storage modulus (G') of the material and the I10 index was also found, whereby a low G' corresponded to a high I10 index. Greater understanding of the relationships between viscoclasticity and pore/fissure network development could enable more precise relationships to be developed, ultimately leading to improved methods for predicting coke quality and devising strategies to make high-quality coke from various sources.