In this paper, the factors controlling the hydrocoprocessing of coal and waste tires in tubing bomb reactors are studied. The fixed experimental conditions were 400 degreesC and 10 MPa of H-2 pressure. Five different coprocessing times (60, 30, 10, 5 and 3 min) and two coal-tire ratios (80% coal-20% tire and 20% coal-80% tire) were the studied variables. Results are compared with those obtained when coal and tire were processed by isolate. At these conditions, it was observed that reaction time has no effect on tire conversion in obtaining the possible maximum value. However, with coal, reaction time is a fundamental variable because total conversion depends on it. In tire, reaction time has no effect on total conversion, but it affects product distribution in obtaining lower oils yields at longer reaction times due to hydrocracking reactions. In hydrocoprocessing of both materials, a synergism in asphaltenes formation is observed at short reaction times. This synergism has been explained by the inter-reactions between coal and tire radicals. Besides, tire addition to coal hydrogenation processes improves the quality of the oils. (C) 2001 Elsevier Science B.V. All rights reserved.