Gas holdup data were obtained for three different kinds of coals in three coal liquefaction pilot plant reactors connected in series of the 150 ton-coal/day NEDOL process. The gas holdup measured was correlated with the superficial gas velocity calculated by taking into account the vapor-liquid equilibrium in the liquid or slurry phase at operating temperatures and pressures. The results showed that the gas holdup increases almost linearly with the superficial gas velocity in a range of 0.07-0.08 m/s and levels off with a further increase of the velocity. Little influence on the gas holdup was observed for the coal rank, the reactor temperatures ranges from 322 to 731 K and the pressures ranges from 16.8 to 18.7 MPa. Also, a comparison of the present data with those reported for other liquefaction reactors having diameters greater than 0.175 m indicated little effect of the diameter on the gas holdup. From the analysis of the data on the basis of the drift flux model, the flow in the reactors appears to fall in a homogeneous bubble flow regime within the range of the superficial gas velocity employed in the present experiment. Further, an empirical correlation between the gas holdup and the superficial gas velocity was obtained on the basis of the slip velocity concept for a homogeneous bubble flow. (C) 2001 Elsevier Science Ltd. All rights reserved.