During the sixth quarter, work was completed on modifications of the high pressure reactor (HPR) system for higher pressure (600 psig) operation. A series of pyrolysis experiments was completed in the HPR using mixtures of nitrogen and oxygen at various pressures and compositions. The original intention of the O/sub 2//N/sub 2/ experiments was to examine the effects of oxygen on the pyrolysis product distribution and the char reactivity. Unfortunately, at the temperature, residence time and pressure used for most of the experiments in this series (817/sup 0/C, 0.47 s, 26 psig) nearly complete oxidation of the char and volatile products occurred. Additional CO/sub 2/ gasification experiments were done to provide a database for revising the test plan. These were done at a variety of temperatures, pressures and CO/sub 2/ concentrations. The reactor was then switched back to nitrogen and additional pyrolysis experiments were done at high pressure (300 psig). When it was observed that the material balance was falling off slightly, it was decided to repeat some of the previous pyrolysis experiments at low pressure. There appeared to be a small, but systematic, reduction in char recovery. We continued to do char reactivity and surface area measurements for chars using the new non-isothermal technique described in the Fifth Quarterly report. A comparison was made of the results for char, tar, and gas yields for 1 atm and 5 atm (60 psig) experiments in the HTR. The asymptotic yield of about 10% was similar for both sets of experiments. A comparison was also made of tar yields for the Montana Rosebud coal from experiments in the HPR and HTR. The observed tar yields were in good agreement for similar experimental conditions. A preliminary version of a particle temperature model for the HPR was completed during the past quarter. 9 refs., 2 figs., 3 tabs.