The molecular weight of a number of vacuum residue and their SARA fractions (Saturated, Aromatics, Resins, and Asphaltenes) typical of the Barrancabermeja refinery in Colombia were determined using the following methods: Gel Permeation Chromatography (GPC), Vapor Pressure Osmometry, (VPO), and Simulated Distillation at High Temperature adjusted to the Gamma distribution function. Molecular weight results in vacuum residue and saturated and aromatics fraction obtained from the three techniques used show consistency within an acceptable range. However, the molecular weight results in resins and asphaltenes fraction obtained by the VPO technique significant differences compared with the results found using the techniques of GPC and DS-Gamma.