Abstract Published exponential equations represent relative concentrations of C 3 - n -C 5 and C 7 + components in petroleum. An additional exponential compositional regularity is presented, involving ethane, propane, pseudo-butane ( i -C 4 + n -C 4 , P4) and pseudo-pentane ( i -C 5 + n -C 5 , P5), as well as a review concerning reaction mechanisms. Pyrolysates of kerogens, petroleum asphaltenes, and model compounds exhibit both series, C 3 - n -C 5 and C 2 –P5, simultaneously. Examination of 400 reservoir fluid (PVT) analyses of oils shows that both light end exponential progressions are closely approached simultaneously in 23.5% of the set, while 10% pass stringent statistical testing, despite commonplace, demonstrable, alteration which partially or completely destroys the relationship. It is shown that the occurrence of C 2 –P5 exponential progressions indicates that cracking occurs almost exclusively by free radical mechanisms. It is also concluded that n -alkenes so generated are partially catalytically converted to carbenium ions which are subsequently isomerized to branched alkenes. In numerous instances proportions of branched compounds are close to thermodynamic equilibrium. The relationships described contribute to the ultimate goal of calculating petroleum compositional and property data, particularly gas-oil ratio and saturation pressure, on the basis of the extent of cracking of methylenic precursors. Slope analysis assists in determining whether or not the C 2 -P5 complement is autochthonous or altered by gas addition or depletion, considerations of potential value in assessing both regional- and reservoir-scale migration history, and in resource appraisal.