Infrared and NMR spectroscopy and thermogravimetric analysis were employed to investigate the structural changes in the Arab heavy and Arab medium asphaltenes following the Friedel-Crafts alkylation and permanganate oxidation reactions. The results provided significant and in-depth information regarding the nature of chemical reactive sites present in asphaltenes. This work gave clear indication that the aromatic C-H group had undergone alkyl substitution reaction in presence of strong Lewis acid AlCl3. The CH2/CH3 groups oxidized to carboxylic acids and other carbonyl functional groups during the oxidation process. Since asphaltenes possess long aliphatic chains, condensed aromatic rings, heteroatoms and metals, information obtained from the structural changes during these reaction processes is considered very significant. The combinations of IR and NMR spectroscopy and TGA have proven to be very useful tools in assessing the structural changes occurred in the asphaltenes on subjecting to oxidation and alkylation reactions. (C) 2003 Elsevier Science Ltd. All rights reserved.