Molecular weight distributions of Green River, Rundle, and Athabasca bitumens and two Athabasca bitumen products were measured using Cf-252 plasma desorption mass spectrometry (PDMS). All samples showed continuous molecular weight distributions up to about 3000 amu except Rundle bitumen. Its PDMS spectrum displayed a trimodal molecular weight distribution. Number average molecular weights calculated from PDMS spectra were compared to those obtained by vapor pressure osmometry (VPO) in toluene or THF. Average molecular weights measured by the two methods agreed well for the three bitumens and did not agree well for the bitumen residues. The PDMS measured molecular weight distributions of Athabasca bitumen samples were compared with molecular weight distributions measured by a combination of gel permeation chromatography (GPC) and VPO. The molecular weight distributions agreed reasonably well at high mass but differed significantly at low mass. Cf-252 plasma desorption mass spectrometry may be a useful rapid technique for determining bitumen molecular weight distributions.