Chemical characterization of heavy oil fractions is impractical because of their complexity. As an alternative, thermogravimetric analysis (TGA) provides fast and valuable information about composition of streams entering and leaving visbreaker unit in oil refineries. Deconvoluting thermogravimetric curves allows thermal-based composition of visbreaking streams to be elucidated by using pseudo-components: four for visbreaking feed and seven for visbreaking residue. In both cases, some pseudo-components explains volatilization of light substances (viz. naphtha or gasoil) under 350 degrees C, and three explains cracking of heavy molecules (viz. resins or asphaltenes) around 380 degrees C, 411 degrees C and 453 degrees C. Obtained results make it possible to define a thermogravimetric index that characterizes composition of visbreaking samples (feed and residue). This index is higher in feed than in residue, and it is higher in light samples than in heavy samples. Deconvolution of TGA curves was improved by using autocatalytic kinetic based on extended Prout Tompkins equation. (C) 2016 Elsevier B.V. All rights reserved.