Crude oils consist of tens of thousands of components belonging to various hydrocarbon families, including n-alkanes, cyclo-alkanes, and aromatics. Because the experimental determination of the composition and molecular structure of each component is an infeasible task, it is common to use characterization factors that describe the nature of the pseudo fractions based on their bulk thermophysical properties. In this work, a new characterization factor, called the aromatic ring index (ARI), is developed based on measurements of molecular weight and refractive index. Unlike other popular characterization factors (e.g., Watson's K-w), ARI can clearly distinguish between various hydrocarbon families. Additionally, and most importantly, ARI can provide a quantitative measure of the aromaticity of the hydrocarbon of interest as it provides a rough indication of the number of aromatic rings contained within the molecular structure. Applications of the new ARI concept to well-defined mixtures of hydrocarbons as well as true boiling point (TBP) distillation data of bitumen and heavy oils indicate that ART shows reasonable trends consistent with the current understanding of the nature of petroleum fractions. Additionally, the concept of ARI can be used to explain some of the apparent discrepancies in the reported results from popular characterization methodologies for asphaltenic systems. Unlike other measures of aromaticity, ARI shows a clear monotonic trend in the aromaticity of the saturates-aromatics-resins-asphaltenes fractions with asphaltenes as most aromatic, which is consistent with the current understanding of asphaltenes.