Pyrolysis of 34 coal's of different rank-with carbon contents of 73.7-91.9% was studied in a TG-MS system to analyze CH4 and H-2 evolution. On the basis of the thape of evolution curves and dissociation energy of covalent bonds in coals, the CH4, formation is attributed to the cleavage of a C-al-C-al bond in a low temperature range and that of C-al-C-ar bond in a high temperature range, while the H-2 formation was attributed to the deavage of H-C-al bond in a low temperature range and-that of Cal-Car bond in a high temperature range. The yield's of CH4 and H-2 corresponding to cleavage of each of these bonds are quantified-by deconvolving each of the evolution curves into two subcurves. Itis found that the cleavage of bonds containing only aliphatic-carbon, such as C-al-C-al and H-C-al peaking at 549 and 540 degrees C, respeCtively, contributes to small fractions of CH4 and H-2 generation. The cleavage of bonds containing an aromatic carbon, such as Cal' Car and H Car-peaking at temperatures of 618 and 774 degrees C, respectively, contributes to the major CH4 and H-2 generation. The yields of the products and the proportion of each bond in the coals that cleaved to 'generate these products are found to be coal rank dependent. The changes of side-structure linked to these bonds are also analyzed with respect to changes in coal rank. These results extend the understanding on the mechanism of CH4 and H-2 formation in coal pyrolysis as well as that on the bonding structure of coals.