In-situ ESR (Electron spin resonance) spectroscopy of free radicals is used to investigate the coprocessing of Blind Canyon coal with Michelin tire rubber for temperatures up to 480 degrees C and under H-2 and argon pressures up to 1000 psig. Comparing the free radical intensities of the coal and the tire alone with those obtained for the 1:1 coal-tire mixtures, determined under 500 and 1000 psig of H-2 and under 1000 psig of inert argon, the following observations are made. First, a strong synergism between the coal and the tire polymer is indicated, as the intensity of the ESR-active free radicals in the coal-tire mixture under H-2 pressure are enhanced severalfold beginning at approximate to 200 degrees C and peaking near 350 degrees C. Second, increasing the H-2 pressure from 500 to 1000 psig reduces the free radical intensity, most likely because of enhanced hydrogenation. Third, the tire polymer gives a weak ESR-active signal whose characteristics change dramatically between 140 degrees C and 200 degrees C, indicating a phase change in the tire material. The synergistic effect observed here is consistent with the reported improved yields in direct liquefaction experiments using coal-tire mixtures.