The purpose of this research was to study the influence of 2 wt% of polyethylene terephthalate (PET) on carbonization of two bituminous medium-volatile coals being different in maximum fluidity (MF). During the research into consideration were taken the structure of the coal plastic layer, the value and the distribution of the intralayer pressure, the changes in volume of the heated coal charge and in the development of the porous structure of the coke, so the changes in electric and dielectric properties of the solid residues obtained from the co-carbonisation of the coals with PET and a coal-tar pitch. The investigations were carried out in a laboratory unit using X-raying and tracing the coal charge with markers of copper foil. Changes in the porous structure of the carbonized coals were estimated on the basis of micrographs taken with a scanning electronic microscope (SEM). It was established that under the influence of PET the thickness of the plastic layer of coals decreases; its zone structure modifies, value and the distribution of the intralayer pressure in the heated charge of bituminous coals changes. PET can change the mechanism of formation of coke obtained from coals with a lower MF index, which leads to appearance of a less dense coke. PET facilitates the formation of a denser coke when blended with a coal with higher MF. The change of electric and dielectric properties of co-carbonisation products of coals with PET and coal tar pitch is analysed. The analysis shows that PET reacts with coals with distinct MF indices in a different way and has an influence on them opposite to that of pitch. Namely: when blended with a coal of low MF, PET facilitates the formation of cross-links between the macromolecules, and with high MF-the processes of macromolecular chains' growth due to the increase in their length. (c) 2006 Elsevier Ltd. All rights reserved.