fragmentation of various mercury compounds of the types HgR2, HgXR and HgX2 is reported where R = Me, C6H5, mFC6H4, p-FC6H4, Me3MCH2 (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 arylmercurichalides the base peak is the carbonium ion