Artificial coalification of a sample of fossil wood of lignitic rank by a confined system pyrolysis in gold tubes reveals that such a simulation only partially mimics natural coalification processes. Analysis of gaseous products by gas chromatography, Liquid products by gas chromatography/mass spectrometry, and solid products by elemental analysis, quantitative solid-state C-13 NMR, and flash pyrolysis/gas chromatography/mass spectrometry allows mass balance calculations. It is clear that, for this sample, confined system pyrolysis reproduces well the natural transformations of aromatic rings, but the reactions of aliphatic structures, derived predominantly from the side-chain carbons of lignin, are not simulated by such an artificial maturation process. This partially explains the deviations from the normal van Krevelen diagram observed in most artificial maturation experiments. The primary gaseous product, CO2, appears to originate from carboxyl and carbonyl groups and the methane, generated during the most severe stages of thermal treatment, appears to derive from methyl substituents on aromatic rings. The main Liquid products observed are phenols, alkylbenzenes, and naphthalenes, with the phenolic products predominating at low severity and the naphthalenes and alkylbenzenes at high severity.