This paper summarizes the experimental work performed on a bench-scale pre-pilot unit for investigating hydrogen mass transfer and mixing energy effects in SRC-II coal liquefaction reactors. Experimentswere carried out with an Ireland Mine coal where the effects of mixing energy level (150-I 000 rpm), method of hydrogen introduction (preheater flow and direct reactor sparging) and hydrogen treat rate (4 to 6 g of hydrogen/l 00 g of feed slurry) were investigated. Several runs using Powhatan No. 6 coal were also carried out where the effect of mixing energy level (200-1000 rpm) was investigated. Other run conditions werefixed to correspond to those likely to be used in commercial operation. The experimental resultsclearly indicated that below a mixing energy level corresponding to400 rpm a significant cementlike solid deposition within the reactor (hydrogen mass transfer limitation) occurred. Below this mixing energy level the Cs+ liquid yield decreases, and the selectivity of the reaction changes, resulting in an increase in the C-C, yield, Thiscritical mechanical mixing level corresponds to a mixing energy per unit of reactor volume of a3500 ergs/cm3 s (350 watts mW3). For the run conditions employed, increasing the preheater hydrogen flow from 4 to 6 g of Hz/1 00 g of slurry prevented the formation of solid deposits at a mechanical mixing energy level as low as that corresponding to 200 rpm. Furthermore, the highest C,+ yield in the entire data set occurred when the preheater hydrogen flow was at the higher level.