Asphaltenes derived from the condensation of maltenes under high-pressure N-2 and supercritical water (SCW) environments were characterized with various approaches. The reaction kinetics of the condensation of asphaltenes under hydrothermal environments were also measured. By improving the diffusivity of hydrocarbon species and the isolation of aromatic carbon radicals from hydrogen donors in the SCW phase, the dealkylation of alkyl substitutes and the condensation of aromatic rings involved in the condensation of maltenes are promoted. Consequently, asphaltenes formed in the SCW phase (As-SCW) have a higher aromaticity and lower alkyl and naphthenic fractions than asphaltenes formed in the oil phase (As-oil). Nevertheless, As-SCW and As-oil both present a cokelike supermolecular structure with the stacking of polycyclic aromatic rings. Benefitting from the highly fused aromatic molecular structure and the cokelike supermolecular structure, the condensation of As-SCW to coke occurs readily under hydrothermal environments, significantly faster than that of As-oil.