Hydrogenobalam in (metal-free vitamin B12) and cupribalamin are characterized by their fast atom bombardment (FAB) mass spectra which show molecular ion and characteristic fragment ion peaks. These spectra and the high-resolution FAB mass spectrum of cobalamin (vitamin B12) show that the (M+H)+-CN-59 peak for the latter is due to loss of acetamide and not of the central cobalt atom. In the FAB mass spectrum of cupribalamin metal clusters are observed