The characterization of crude oil involves dividing the oil into pseudocomponents and allocating mole fractions, molar mass, specific gravity, average boiling point. and critical properties to each component. The characterization is typically based on distillation data reported in terms of true boiling points. Standard assay types such as the ASTM D86 or ASTM D1160 vacuum distillation do not provide well established Saturated bubble temperatures and require empirical interconversion Curves to convert the assay data into true boiling point (TBP) data. Recently developed assays such as the ASTM D5236 and Bruno's new distillation assay methodology do provide well-defined saturated bubble temperatures that correspond to actual thermodynamic state points but lack ail established interconversion method to a TBP, that is, it method to determine the TBP of the fluid based oil the measured temperatures of the assay. In this work. a methodology is presented to determine pseudocomponent mole fractions that match the boiling point data from these new assays. The fluid is divided into pseudocomponents of different average boiling point, and the molar mass and other physical properties of each component are determined using established correlations. A simulation of the distillation is optimized to match the assay data by adjusting the mole fraction of each pseudocomponent. The characterization can also be constrained to match other data such as the bulk density and molar mass of the fluid. The proposed methodology is tested on naphtha and Alaska crude oil and then verified through three heavy oil case studies. The methodology is entirely general and can be applied to a compositional analysis from a distillation of any material.