1H- and 13C-NMR data were used to characterize asphaltenes and to follow their chemical changes when they were exposed to thermal cracking under different thermal conditions (673, 693 and 713 K), and treatment times (10, 20 and 30 minutes). Samples of asphaltenes were obtained from a Vacuum Residue (VR) of a mixture of Colombian crude oils. Samples were previously characterized by elemental analysis. Since the characterization of the aromatic substructure is important for this work, special attention was given to those regions in the 1H- and 13C-NMR spectra that showed the main changes: p.e. methyl hydrogens -in gamma position or more- to aromatic units HCH3; methyl hydrogens -in beta positions- to aromatics units HCH3; methylene hydrogens -in beta position- to aromatics HCH2; hydrogens -in naphthenic units beta- to aromatics HN; hydrogens -in paraffinic and alpha position naphthenic structures-in - to aromatics HP,N; hydrogens in monoaromatic units HAr; hydrogens in polyaromatics HAr; carbons in methyl groups CCH3; carbons in methyl groups -in alpha position- to aromatics CCH3; protonated aromatic carbons CAr; pericondensed aromatic carbons CAAA; aromatic carbons linked to methyl groups;CAr catacondensed aromatic carbons CAA and aromatic carbons linked to paraffinic or naphthenic structures CAr.