Mass spectrometry plays a crucial role in the structure elucidation of new sterols, notably those of marine origin. Secure interpretation of the mass spectra requires an adequate understanding of the mechanism of the key fragmentation processes of steroids and the present paper summarizes recent work from the author's laboratory dealing with the elucidation of the fragmentation mechanism of the most important peaks of the following classes of steroids: (a) satrated sd hydrocarbons with the l4c

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... d lstereocjeistry; (b) and A -unsaturated sterols; (c) -, A ' -and A ' -unsaturated 3-ketosteroids; (d) sterols with unsaturated side chains. The techniques of deuterium labelling and of metastable defocussing proved to be indispensable in these studies.