Liquid holdup has been determined by means of a self-designed method in an upflow cocurrent bubble column reactor for the purpose of providing an efficient reactor for continuous homogeneous hydrogenation of nitrile butadiene rubber (NBR). The experimental fluids were gaseous hydrogen and chlorobenzene, solution of NBR, analogous to that of NBR hydrogenation in an industrial;process,except without using a catalyst. The influence of the gas and liquid superficial velocities and pressure on liquid holdup under NBR hydrogenation conditions has been investigated. The results indicate that three flow regimes exist, depending on the operation conditions employed. A correlation of liquid holdup, which covers those three regimes, has been established with satisfactory agreement by means of a. modified dimensionless drift flux against the relative flow velocity of gas to liquid.