An atmospheric residue from Athabasca bitumen sweet blend has been characterized using size exclusion chromatography, UV-fluorescence spectroscopy and mass spectrometry (GC-MS, heated probe-MS and laser desorption MS) to indicate the extent of the mass range of the material. The Athabasca bitumen is a more difficult sample than the Maya crude and should provide a more stringent test of the methods applied to the Maya. Fractionation of the sample was by solvent solubility into heptane solubles (maltenes) and toluene solubles (asphaltenes). The maltenes were separated into acetone-soluble and -insoluble fractions; the asphaltenes were separated into NMP-soluble and -insoluble fractions. TLC was used to separate these fractions before LD-MS. LD-MS and SEC gave mass range estimates increasing from maltene acetone-solubles through NMP-insolubles of asphaltenes up to at least m/z 10,000. The fractions containing the largest molecules had little or no fluorescence; fractionation proved essential to detect the largest molecules.