The electron ionisation mass spectra (EI-MS) of oxo- and thio derivatives of minor tobacco alkaloids and structurally similar piperidine alkaloids, i.e. thiocotinine (1), 2′-oxo-N-methylanabasine (2), 2′-thio-N-methylanabasine (3), 2′-oxoanabasamine (4) and 2′-thioanabasamine (5) are discussed and general fragmentation routes of their molecular cations are proposed. Comparison of the data obtained for1with the EI-MS data of metameric metabolites of nicotine: 3′-hydroxycotinine, (A) and 5′-hydroxycotinine, (B) allows a differentiation between these metamers. The data will be useful for the identification of metabolites of alkaloids of these types in biological matrices.