Previous experiments carried out on several carbonization phases showed a relationship between tar migration and the coking pressure, In the present study, tests analogous in principle to the previous ones were conducted at a pilot scale with 400 kg coal charges. Two coals were used: C28, which leads to no pressure (＇＇safe＇＇), and C19, which induces pressure (＇＇dangerous＇＇). N-methyl-2-pyrrolidinone (NMP) extractions from the carbonization phases of the two coals confirm that tar migration is dependent on the nature of the parent coal, i.e. whether ＇＇safe＇＇ or ＇＇dangerous＇＇, In the case of dangerous coal, the impregnation of non-coked coal by tars has been evidenced, In relation to this phenomenon, increased coking pressure is likely to develop due to the enrichment of non-transformed coal by volatile matter, as well as to the drop in permeability of this phase, It is also suggested that heavy tars clog up the pores of the semi-coke of dangerous coals.