The fluid phase which forms during carbonization of a range of coals was studied using rheometry and 1H NMR spectroscopy to study the mechanisms behind the generation of excessive wall pressures during coking. It is proposed that high coking pressures are generated for low volatile matter coals when the temperature of maximum fluidity (Tmf) is >465 °C, the minimum complex viscosity (η*) is > 105 Pa s, the percentage of mobile 1H (fluid phase) is <40%, and the 1H mobility in the fluid phase is <65 μs. It is suggested for these coals that the particles fuse to form a rigid network containing pockets of fluid material which have a low fluidity and do not link up. This arrangement could present an impermeable barrier for gas flow and force the gas to the coal side where it builds up in a diminishing region to a critical level, causing pressure on the walls. The magnitude of the pressure generated may be proportional to Tmf and η* and inversely proportional to the percentage and mobility of mobile 1H. It was also found that a high volatile coal which formed a highly fluid phase over a broad temperature range gave rise to a significant coking pressure. In this case, it is proposed that the sheer expansion of the coal charge as it converts to gas and liquid phases is the reason for pressure on the oven walls. These proposals agree with current thinking on the generation of coking pressure. This work is based on the testing of only nine samples and further work is planned to gain a greater fundamental understanding of fluidity development from which models for predicting coking pressure and coke quality for coal blends may be developed.