Liquid-phase co-thermolysis of waste plastics (e.g. polystyrene, polycarbonate, polyurethane, polyvinyl chloride, tire rubber, unsaturated polyester resin, melamine-phenol formaldehyde resin, polyethylene, polyamide, polyethylene terephthalate, phenol formaldehyde resin) with coal-tar pitch at temperatures up to 400 degrees C yielded gases, pyrolysis oils, and high-melting pitches (reaction pitches). Thermogravimetric analysis of these pitches showed that they were thermally stable up to the plastic temperature range of coals. Hence, the reaction pitches could be used as reactive additives for weak-coking coals to improve their coking properties.