The protonation of acrylonitrile (1) and of Z- and E-3-aminoacrylonitrile (5 and 9) was studied with the Gaussian-2 method. Ammoniumacrylonitrile ion formation is not important. Nitrilium ion formation is preferred in all cases, and the proton affinities are about 80 kJ/mol higher for aminoacrylonitrile. Remarkably, it is found that C2-protonation of 3-aminoacrylonitriles to form iminium ions can compete with nitrilium ion formation. β- Aminoacrylonitriles thus show propinquity to both acrylonitriles and enamines, and mechanistic and toxicological implications are discussed.