This communication describes changes in the macromolecular structure of a lignite and a bituminous coal during rapid pyrolysis in the temperature range 300 to 1200 K. Solvent swelling techniques have clearly demonstrated that crosslinking occurs in lignites at somewhat lower pyrolysis temperatures than it does in bituminous coals. The onset of the crosslinking processes in bituminous coals coincides with the end of the tar formation period. In lignites, crosslinking occurs very early in the pyrolysis process, coinciding with low temperature release of CO/sub 2/. The presence of natural moisture in the lignite appears to have a significant effect on pyrolysis chemistry, increasing the amount of crosslinking observed at any temperature. 14 refs., 6 figs.