Molecular structural characterization of the SCFE narrow-cuts of various residua was made by using molecular connectivity index (MCI) method in order to predict their physical properties and chemical reactivity. First-order MCI 1X was calculated for hundreds of SCFE narrow-cuts and correlated with their physical properties.The MCI 1X of SCFE narrow-cuts correlated very well with their physical properties for a given residue. The predicted physical properties of the SCFE narrow-cuts from various residua varied dramatically although they had the same MCI 1X.Apparently only the carbon atoms in the molecular skeletons are considered for calculating MCI. The existence of hydrogen atoms and their effects on the molecular structure and properties are neglected. Subsequently

the MCI was modified by integrating the atomic ratio of hydrogen to carbon (H/C) into the MCI to characterize the SCFE narrow-cuts of vacuum residua appropriately. A modified MCI KM was proposed by combining H/C and 1X. The quantitative structure property relation (QSPR) derived from the modified MCI KM and residue properties was developed to predict the physical properties and chemical reactivity appropriately. This MCI method was applied to the residue characterization successfully.