For a range of coals varying in the internal gas pressure they generated in a small oven, a small coking reactor was used to measure the volumes of gaseous volatiles which were released from the coal and coke exits of the reactor as the coals were carbonized. Simultaneous measurements were made of either the internal gas pressure or the pressure necessary to maintain a constant Bow of nitrogen into the centre of the charge. Cases initially exited from the coke-side exit, but the direction of gas exit changed when the plastic layer neared the coal end of the charge, the effect being observed for all the coals examined. Internal gas pressures measured in the coking reactor placed the coals in the same order of pressure generated as the small oven. Nitrogen pressure peaks were generally higher than directly measured internal gas pressures and remained measurable to higher temperatures. The results are discussed with regard to the mechanism of coking pressure generation.