The aim of the study is to determine the apparent fate constants for methane generation from the 9-methylphenanthrene (9-MPh) thermal cracking in the temperature range 375-450 degrees C under a constant pressure of 120 bar to evaluate by extrapolation the thermal stability of methylated aromatics of crude oils in geological conditions. In the first step, based on the conversion of 9-MPh only, rate constants were determined and the generated products were identified: the overall kinetic scheme was compared to that proposed in the literature for the l-methylpyrene thermal cracking (Smith, C. M.; Savage, P. E. Energy Fuels 1992, 6, 195-202). Then the methane generated during primary and secondary cracking of 9-MPh was quantified, and the corresponding apparent rate constants for its production were determined. The apparent kinetic parameters for the degradation of 9-MPh are 49.0 kcal/mol for E and 4.5 x 10(10) s(-1) for A, whereas E is 54.5 kcal/mol and A is 1.1 x 10(12) s(-1) for methane generation. When these results are extrapolated to geological conditions (T < 200 degrees C), the Arrhenius diagrams corresponding to the two reactions cross that of n-C-25 between laboratory and geological conditions. Consequently, both the early transformation of the methylated compounds and the late methane production will take place before the occurrence of a significant cracking of the n-alkanes. The thermal cracking of 9-MPh was also compared to that of the lumped chemical class of methylated aromatics generated during artificial maturation of kerogens representative of the main types of organic matter (I, II, and PPI) in closed pyrolysis systems.