Pulverized Yallourn and Taiheiyo coals were mixed with a fine powder of homo-polypropylene, low-density polyethylene or high-density polyethylene. Then the mixture was heat-treated above the melting point of the polyolefin, and composite particles prepared were subjected to flash copyrolysis in a Curie-point pyrolyzer. The copyrolysis decreased total char yield and increased total tar yield for all combinations of coals and polyolefins in comparison with the sum of corresponding yields that were obtained by separate pyrolysis of each component. The promotion of tar evolution was well explained by the transfer of hydrogen and hydrocarbon radicals from polyolefin to coal and by the suppression of cross-linking formation in the coal. These reactions were confirmed from the decrease in yields of hydrogen, paraffinic and olefinic hydrocarbon gases, and inorganic gases. The increment of tar yield was dependent on the temperature of copyrolysis and the coal-polyolefin combination and was well correlated by the decrement of gas yield on a molar basis. Flash pyrolysis was also performed in an entrained-flow pyrolyzer, where secondary pyrolysis in the gas phase occurred. Increase in yield of light aromatics was characteristic of the secondary pyrolysis of initial tar, and unsubstituted aromatics such as benzene and naphthalene were produced more abundantly than substituted ones.