Slurry phase residence time in the SRC-II dissolver is a major process variable affecting the extent of conversion to distillate products. Kinetic studies have shown that overall conversion of vacuum bottoms to lighter hydrocarbons is a zero order process with respect to vacuum bottoms. This implies that although the extent of backmixing is not a critical process variable, slurry phase mean residence time is. Experimental results from the P-99 PDU supported this. Radioactive tracers were injected into the Ft. Lewis dissolver during normal operation to study the residence time distribution of the gas and slurry phases. Results from gas phase radiotracers showed the gas phase to approach plug flow behavior, but with much larger than expected gas holdup. Gas holdup appeared to be 20% of the reactor volume, rather than the 10 to 12% predicted by standard holdup correlations. Slurry phase radiotracer results indicated that the slurry phase was well-backmixed. Experiments with a downcomer designed to enhance direct slurry flow from the top to the bottom of the dissolver showed this to be a viable and controllable device. The high level of gas holdup at Ft. Lewis suggest that standard holdup correlations must be modified to be used with confidence in the design of coal liquefaction reactors. Additional programs to define holdup relationships in high temperature, high pressure coal liquefaction reactors are recommended. 11 references, 6 figures, 6 tables.