A detailed knowledge of the molecular mass (MM) distribution in coal and its derived products is essential for a fundamental understanding of coal structure, and of the processes occurring during coal conversion. Fractionation using size exclusion chromatography (s.e.c.) using N-methyl-2-pyrrolidinone as the mobile phase has been applied to such materials and has provided improved MM distributions. Absolute calibration has been provided using matrix assisted laser desorption ionisation mass spectrometry (MAlI-m.s.). An alternative method of volatilising and ionising large molecules for mass spectrometry (m.s.) is Cf-252 plasma desorption (Cf-252 p-m.s.). This involves the use of energetic fission fragments from the decay of Cf-252 and produces mass spectra consisting predominantly of molecular ions from a range of polymers and biomolecules. This has been used by other workers to determine the molecular weight distribution of heavy distillation residues obtained from coal liquefaction processes either unfractionated or fractionated into broad fractions. Generally, a good agreement was obtained between values of MM determined by Cf-257 p-m.s. and s.e.c.. A comparison is reported of MM distribution determined by Cf-252 p-m.s. and laser desorption mass spectrometry (l-m.s.) for narrower fractions separated by s.e.c. from a coal tar pitch.