The overall objective of this research is to obtain, for conditions pertinent to modern coal gasification practice, improved basic understanding of coal plasticity, coal particle agglomeration, and secondary reactions of freshly formed coal pyrolysis tars. Development and construction of a final, first generation version of a high temperature, fast response coal viscometer have been completed. The viscosity of a Pittsburgh coal under a heating rate of a few hundred K/s was found to be in the range 10/sup 2/ to 10/sup 4/ poises. A systematic study of homogeneous and heterogeneous secondary reactions of tar has been completed, using a two-stage (generation/cracking) tubular reactor. The system contains two independently heated tubular reactors connected in series. A thin bed of coal, diluted with sand to prevent agglomeration, is pyrolyzed in the upstream reactor (reactor-1) at a low heating rate (3/sup 0/C/min) in order to produce fresh tars at temperatures (< 500/sup 0/C) unfavorable to secondary reactions. These tars (and other volatiles) are then swept with carrier gas, usually helium, to the downstream (reactor-2) stage which is held isothermal at a temperature between 500 and 900/sup 0/C, and then to product traps. In the second stage, the separate effects of temperature (500 to 900/sup 0/C) and residence time (0.6 to 1.1 s) on homogeneous and heterogeneous tar secondary reactions were studied at 0.3 to 1.3 atm (gauge) pressure in helium. Selected runs were done at longer residence times (up to 3.9 s) for homogeneous experiments and in CO/sub 2/ atmospheres (1 to 5 atm, gauge) for heterogeneous experiments with active carbon. Gases and light oils were analyzed by gas chromatography. Tars were analyzed by H-NMR, gel permeation chromatography, column chromatography, and elemental analysis. Surface areas and pore size distributions of char and active carbons were respectively measured by N/sub 2/ adsorption and Hg porosimetry. Results are discussed. 61 references, 35 figures, 5 tables.