Oxidative pyroanalysis apparatus was settled to fully characterize heavy oils. Fractions distribution, and elemental composition were obtained in one single run on both volatile and non volatile fractions. Correlation between experimental Carbon residue values and Conradson carbon residue data is possible and a crackability index of the heaviest part of heavy oil is determined. From all gathered data, quality of products can be appreciated in these circumstances as well as thermal sensitivity resulting in accurate determination of conversion indices.