A plausible mechanism is provided for the generation of internal gas pressure (IGP) in the plastic layer of a coke charge, by relating the formation of low permeability at the outer edges of the plastic layer to the deformability of the plastic material. The amount of lateral shrinkage post-resolidification is shown to be important in constraining the deformation of the plastic material, and hence the generation of low permeability to gas flow. The mechanism provides a natural explanation for permeability variation in the plastic layer, as well as for the linkage between IGP/coking pressure generation and pushing difficulties.