The technique of mass-analysed ion kinetic energy spectrometry (MIKES), using a mass spectrometer in which ions are selected according to their mass to charge ratios before entering the electric sector, is shown to add a new dimension in determining the structures of unknown compounds. Several kinds of additional analytical information over that obtainable by conventional mass spectrometry are available. The use of a collision gas to induce fragmentation further extends the usefulness of the method. Each of the reasons for this is discussed and illustrated separately by the examination of two compounds, the first of which could be shown to contain only carbon, hydrogen and oxygen and the second carbon, hydrogen, oxygen and sulphur.