In this work, the changes in electrical resistivities of a vacuum residuum are analyzed to determine pyrolysis reaction rates at different temperatures during high temperature processing. Fractions of the residual oil from vacuum distillation were treated at a constant and uniform temperature, and electrical resistance was related to aromaticity. The results show that resistivity behaves like a first order decay-like thermal cracking reaction. This method could replace chemical methods used to determine the temperature necessary for thermal cracking reactions and the kinetic parameters of heavy petroleum fractions. (C) 2010 Elsevier Ltd. All rights reserved.