Molecular weight (MW) is one of the most important properties of asphaltene. To determine the precise average MW using gel permeation chromatography (GPC), the MWs obtained for three different asphaltenes using GPC (MWps) and GPC-MSD (MWms) were compared. MWms proved independent of asphaltene type and had values similar to MWps for a MW of similar to 900 but was significantly different in the lower-MW range, with a linear correlation between MWms and MWps. More-precise average MW values can be obtained using a revised calibration curve based on three new chemicals to convert MWps to MWms.