A porphyrin-containing fraction isolated from the > 500 degreesC residue of Chinese Gudao, heavy oil was analyzed using laser desorption ionization-time of flight mass spectrometry (LDI-TOF-MS) for the first time. LDI-TOF-MS was revealed to be suitable for the characterization of petroporphyrins mixture. LDI-TOF mass analysis, combined with UV-vis measurement, indicated that the nickel porphyrins in Gudao residue are a mixture of ETIO homologues (C-25-C-34) maximizing at C-30 and DPEP homologues (C-28-C-37) maximizing at C-31. The SigmaDPEP/SigmaETIO ratio is about 1.04, suggesting Gudao oil is in the stage of becoming mature. (C) 2004 Elsevier Ltd. All rights reserved.