Coals characterized by different volume contraction or expansion (from -28 to +8 mm according to Koppers-INCAR test) and different "wall" pressure (0.4-49 kPa) were analyzed by means of pyrolysis field ionization mass spectrometry (Py-FIMS). The aim was to disclose characteristic features of thermal decomposition of the coals that influence their contraction as well as their "wall" pressure development during coking. The results showed the relationship between contraction values and the "wall" pressure, Py-FIMS results indicate that the contraction values depend on ii) yields of thermal degradation products generated in coals on their heating to resolidification temperature and (ii) individual composition of the thermal decomposition products, The coals show high contraction (Koppers - INCAR test less than or equal to -10 mm) and low wall pressure (<5 kPa) under two conditions : their yields of (i) species are over 14 wt % and (ii) high-alkylated aromatics predominate over low-alkylated aromatics in the Py-FIMS detected decomposition products. It has been concluded that the yield and the composition of the thermal degradation products generated in the plastic layer influence not only the contraction of the semicoke layer but also its permeability for the gaseous and vapor products.