The transformation of the porphyrin-free asphaltene fraction from a heavy Maya crude oil was catalyzed by chloroperoxidase from Caldariomyces fumago (CPO) in a ternary system with low water content of 6.3%. The total turnover number (TTN) in this system was determined for lyophilized CPO in the presence of sucrose as cryoprotector and for a covalent bioconjugate with chitosan. The covalent conjugation of CPO with chitosan increased the TTN by 10-fold. Fluorescence due to aromatic groups decreased by 24% in treated asphaltenes, indicating significant changes after the biotransformation. On the other hand, the solubility profile indicated that transformed asphaltenes are less soluble in toluene and more prone to precipitate in the presence of hexane, compared to untreated asphaltenes, which could be related to the introduction of polar atoms. Energy-dispersive X-ray spectroscopy (EDS) showed that the content of chlorine increased six-fold on an atomic basis. Finally, enzymatically transformed asphaltenes are more reactive and thus generate less coke, as evidenced by thermal degradation under nitrogen atmosphere. (C) 2011 Elsevier Ltd. All rights reserved.