Hydrogen-donor ability has been ascribed as one of the factors responsible for stabilisation of the plastic phase during coal carbonisation. In situ high-temperature H-1 nuclear magnetic resonance spectroscopy (NMR) is used here to compare quantitatively the interactions of a low-volatile, poor coking bituminous coal with a heat-treated coal tar pitch (CTP) and with a hydrogen-donor pitch (HDP) ( > 450 degrees C residue) obtained from a two-stage coal liquefaction process. When the CTP was added to the coal (25% w/w, 150-250 mu m), the amount of fluid material increased by nearly 20% more than that predicted at maximum fluidity close to 450 degrees C. Indeed, an even larger synergistic effect was observed with the HDP. By 400 degrees C, 90% of the fluid phase concentration observed at 450 degrees C had already been generated, corresponding to an enhancement of 50% over that predicted. However, particle size appears to be a dominant factor under the slow heating regime used, in that no enhancement in fluidity was detected when the coal size was < 45 mu m.