Metal species (Ni and V) in as-received atmospheric residues (ARs) were characterized through conventional analysis methods as well as through gas chromatography-atomic emission detection (GC-AED) assembled with a high-temperature injector system (HT-GC-AED). The vanadium concentration in all ARs could be successfully measured by electron spin resonance (ESR) corrected with inductively coupled plasma-mass spectroscopy (ICP-MS). HT-GC-AED was attempted to confirm the characteristics of V and Ni species, from which at least five species of vanadium and six species of nickel in as-received ARs were identified, together with several shoulder peaks. In addition, HT-GC-AED analysis of product oils after hydrodemetallization (HDM) of ARs was investigated over both oxide and sulfide catalysts. This analysis provided evidence of a demetallization mechanism for the cracking of the alkyl group attached to the metal porphyrin structure over oxide catalysts. This mechanism contributes to the enhancement of HDM based on the staged reaction over the oxide (1st stage)-sulfide (2nd stage) catalytic system. (C) 2013 Elsevier Ltd. All rights reserved.