Catalytic amounts of a protic reagent such as tert-butyl alcohol promote the isomerization of a stable amino-aryl-carbene into a transient azomethine ylide. Deprotonation of an alkyl-aldiminium salt also leads to a transient azomethine ylide, but labeling experiments rule out the transient formation of the corresponding amino-alkyl-carbene. The potential hypersurface between model amino-carbene, aziridine, and azomethine ylide is investigated.