Hydrogen exchange has been studied during lignite pyrolysis by heating mixtures of an oven-dried (105 degrees C) Loy Yang lignite and deuterium-labeled water (D2O) in a quartz U-tube reactor. A typical experiment consisted of heating the lignite from ambient temperature to 500 +/- 2 degrees C at 20 degrees C/min under a flow of nitrogen. A higher-heating rate experiment (6 degrees C/s) was also performed. The collected tar fractions were analyzed by gas chromatography and mass spectrometry, which allowed detailed analysis of individual components. Inspection of the mass spectra of some of the components showed changes that reflected deuterium had been incorporated into their molecular structure during pyrolysis. The relative extent of deuterium incorporation was quantitated by mass fragmentography of selected ions, with the molecular ion in most cases being used. Four compounds were chosen for detailed analysis: 1-methylnaphthalene, 2-methylphenol, 1-octadecene, and n-octadecane. The pyrolysis experiments showed that a significant amount of deuterium was incorporated into the aromatic and phenolic components of the tar, while the amount of deuterium incorporated into the 1-alkene and n-alkane components was much smaller. A series of experiments was completed to determine the influence of experimental parameters such as the heating rate, purge gas flow, and amount of deuterium-labeled water present. Additional experiments using in-line mass spectrometry monitored the fate of the deuterium-labeled water during pyrolysis and the generation of chemically produced pyrolytic water. Reaction mechanisms for the observed hydrogen exchange reactions are proposed, and the significance of these to the overall lignite pyrolysis process is discussed.