A sample of Athabasca bitumen was fractionated by preparative size-exclusion chromatography (SEC), and the molecular-weight distribution of the five fractions and the original sample were determined by SEC in NMP solvent using a calibration based on polystyrene standards, by Cf-252 plasma desorption mass spectrometry (PDMS), and by laser desorption and matrix-assisted laser-desorption mass spectrometry (LDMS and MALDI) and the average molecular weights were measured by vapor pressure osmometry (VPO). A comparison of the results shows that VPO, MALDI, and SEC gave similar results while PDMS showed the same trend. The molecular-weight distribution of this 525+ degrees C fraction is continuous up to and perhaps beyond a molecular weight of 15000.