The fragmentation of various mercury compounds of the types HgR 2, HgXR and HgX 2 is reported where R = Me, C 6H 5, mFC 6H 4, p-FC 6H 4, Me 3MCH 2 (M = Si, Ge) and X = halide. Their behaviour, especially that of the organomercury halides, varies considerably with the nature of the R group whilst changing the halide has little effect. For diarylmercurials and arylmercuric halides the base peak is the carbonium ion R + whereas in the methylmercury halides the base peak is HgXMe +·. In compounds containing the grouping Me 3MCH 2Hg the fragmentation is dominated by cleavage of the MeM rather than HgC bonds. For the mercury(II) dihalides variations in electron voltage give evidence for two routes to the X + ion. Attempts to prepare (o-FC 6H 4) 2Hg resulted in the isolation of 2, 2′-(o-fluorophenyl)diphenylmercury. Some ionization and appearance potentials are reported.