The aim of this work was to elaborate a mathematical model that accounts for the carbon isotopic composition of methane generated during the thermal cracking of two model compounds: 9-methylphenanthrene (9-MPh) and 1-methylpyrene (1-MPyr). Pyrolysis experiments were carried out; in an anhydrous closed system (gold vessels) during times ranging from 1 to 120 h under isothermal conditions (400-475 degreesC) at a constant pressure of 150 bar. Global rate constants were determined for methane generation from l-methylpyrene decomposition, similar to those determined by Behar et al. (see ref 36 in the text) for 9-MPh thermal cracking. Two main processes of methane formation were recognized: one related to the loss of the methyl group and the second corresponding to the opening of the aromatic rings, the second of which is hydrogen pressure dependent. The derived apparent first-order kinetic parameters were determined only for the first process: E = 55.6 kcal/mol and A = 4.2 x 10(12) s(-1). These parameters are in the same range as those found for methane generated from 9-MPh (ref 36). When these results are extrapolated to geological conditions, methane generation occurs at temperatures lower than 200 degreesC and, thus, constitutes a significant source for natural gas accumulations. This source of natural gas can compete with late methane generation from kerogen. Based on the global kinetic scheme proposed for methane generation, a model of carbon isotopic fractionation was elaborated for predicting the isotopic composition of methane. Results show that very high isotopic fractionation can take place when the methylated aromatics are thermally degraded: the demethylation reaction leads to an isotopic fractionation between the generated methane and its source which is significantly dependent upon the isotopic heterogeneity of the aromatic compound. This study shows that specific isotopic signatures in natural gas might fingerprint the secondary cracking of aromatics in deep reservoirs.