The objective of this contract is to extend and optimize UOP's single-stage slurry-catalyzed co-processing scheme. Particular emphasis is given to defining and improving Mo catalyst utilization and cost, evaluating alternative and disposable slurry-catalyst systems, and improving catalyst recycle and recovery techniques. During the previous quarter, a catalyst concentration study was completed. The study showed that the highest nondistillable conversions and liquid yields were achieved using liquid recycle at temperatures in the range of 450--460{degree}C. At these high severity conditions, the liquid product yield and light ends yield were nearly independent of catalyst concentration. During the current quarter a follow-up study was conducted without catalyst. The objective of this study was to determine whether the improved high temperature operability was due to improved hydrodynamics resulting from the use of liquid recycle or whether catalyst, also plays a role, even at small concentrations. The results of bench-scale Run 28 are discussed in this report. 1 ref., 12 figs.