The high concentrations of the radiation-generated unstable molecular states necessary for propagation of the low-temperature chain cracking reactions in the petroleum feedstock are interpreted in association with the earlier reported effect of the intense radiation-induced isomerization especially characteristic for the high-viscous oils: The hypothesis of the interconnection between cracking and isomerization reactions is supported by the available experimental data on the dose rate dependence of radiation cracking rate and the data on the kinetics of radiation-induced isomerization. (c) 2012 Elsevier Ltd. All rights reserved.