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• Sulfur compounds were identified based on DBE of SARA fractionations. • S1 class species were the key refractory sulfur compounds in RHT process. • Reactivity of the sulfur compounds in residual oils is structurally sensitive. • Reactivity difference of sulfur compounds in residual oils is small. The molecular structure of sulfur compounds in residual oils and their desulfurization reactivity during residue hydroprocessing (RHT) process were investigated by virtue of the self-established semiquantitative APPI FT-ICT MS combined with the kinetic analysis of the hydrodesulfurization of individual sulfur compounds. In order to increase the characterization accuracy and gain more structure details on the sulfur compounds, a prior SARA separation scheme of residual oils into clearly-defined fractions was performed. Results show that S1, S2, S3, and N1S1 class were the dominated species in the residual oil feedstock. During RHT process, the S1 class species were identified as the key refractory sulfur compounds, while S2 and S3 class species were identified as the easily-removed sulfur compounds. The molecular structure change of S1 class species during RHT process was marked by a drastic increase in the aromaticity and polycondensation, indicating that S1 class species with less-aromatic cores exhibited higher reactivity. Moreover, a novel DBE-based lumping kinetics approach containing 6 sulfur groups of S1 class compounds was developed to investigate the desulfurization reactivity of individual sulfur compounds. The results revealed that the reactivity of sulfur compounds depends on their structure, and different molecular structure brings distinctly different reactivity. The reactivity difference between the individual sulfur compounds in residual oils is considerably smaller than that in other petroleum fractions with lower boiling points, such as VGO, diesel. This research provides a deep insight into complex desulfurization chemistry during RHT process. [ABSTRACT FROM AUTHOR]