Main structural parameters of low and middle rank coals and genesis (C-daf = 64.3-87.2%) were determined by method of X-ray diffraction. The interrelation between structural parameters (L-c, L-a, h/l(002)) and carbon content (C-daf) of coal was found. It is determined that impregnating of coals by 1 M of inorganic substance solution (KOH, CaO, HCl) and water leads to a change in the initial coal structure and is determined by a reagent nature. Data of IR-spectroscopy and X-ray diffraction for parent and impregnated coals suggest that the effect of inorganic substances on coal involves substitution of free acid group protons with metal (K, Ca) cations and a rupture of C-O-C and C-C bonds takes place as well as a rupture of organo-mineral bonds (O-Me-O), sulfide bridges and a destruction of cation-forming complexed (HCl). The influence of extraplanar deformational vibrations of CH2- and CH3-bonds of carbon skeleton with a successive redistribution of aromatic and aliphatic fragments in coal structure is increased. The correlation between the degree of interlayer ordering (h/l(002)) of parent and impregnated coals and the pyrolysis liquid products at a temperature of 500 degrees C was found. It was shown that coals impregnated in the course of pyrolysis increases the liquid products yield, 1.5-2.0 times, in comparison with untreated coals. Thermogravimetric analysis (TGA) data were used to determine the energies of activation (E-ef) for some coals. The correlation between TGA and X-ray diffraction data was found. (C) 2000 Elsevier Science Ltd. All rights reserved.