Mass spectra of products from the liquid phase pyrolysis of 20 hydrocarbons having from one to four aromatic rings and molecular weights from 116 (indene) to 228 (chrysene) were investigated. By studying the pyrolyzates of several structural types found in the material extracted from Pittsburgh seam (hvab) coal with pyridine at room temperature, information related to the formation of heavy residues such as coal tar was obtained. The amount of original material remaining after pyrolysis at 450ยฐC for 4 hr was used to estimate the extent of thermal reactivity. Many of the molecular weights appearing in the mass spectra of the residues could be correlated with structures resulting from the recombination of radicals. The highest mass peaks in the spectra of the pyrolyzates of naphthalene, phenanthrene, and pyrene were 254, 354 and 402, respectively, corresponding to dehydrogenative dimers of these compounds. Thermal treatment of binary mixtures indicated extensive interaction of species derived from the original structures. The residue from the liquid-phase pyrolysis of a 1 :l mixture (by weight) of 9,10dihydroanthracene and acenaphthylene indicated extensive interaction of species having three aromatic rings to produce species having up to seven condensed rings. Mass spectrometric and other data have indicated that the major components in the liquid and solid product from the low-temperature carbonization of coal have molecular weights below about 400. This can be explained if, on pyrolysis, most of the major structural types in the room-temperature extract of coal produce components resulting primarily from the recombination of radicals.