The standard molar enthalpies of formation, at T = 298.15 K, of all possible single methylated derivatives of benzothiophene and dibenzothiophene were calculated by means of the G3(MP2)//B3LYP approach employing several different working reactions (homodesmotic and atomization). The most stable compounds are the 7-methylbenzothiophene and 4-methyldibenzothiophene while the least stable are the 5-methylbenzothiophene and 1-methyldibenzothiophene compounds. Calculated enthalpic increments for the reactions of methylation are in the range -29.5 and - 39.1 kJ mol(-1).