The structures and unimolecular fragmentation reactions of protonated iminopropadienones, RN=C=C=C=O (R=H and CH3), were investigated by a combination of mass spectrometry based experiments (collisional activation (CA), neutralization-reionization (NR), and chemical ionization (CI) mass spectrometry and flash vacuum pyrolysis) and high-level ad initio molecular orbital calculations, at the QCISD(T)/6-311+G(2d,p) level of theory. The C- and N-protonated forms, RN(+)=C-CH=C=O and R(HN+=C=C=O, were separately generated from a variety of precursors. The experiments and calculations indicate that the most favorable site of protonation of iminopropadienones is the central carbon atom (C-2), and the resulting ions are best regarded as N-alkylated or N-protonated cyanoketenes. The N-protonated iminopropadienone is close in energy to the C-protonated one, while the O-protonated form is significantly higher in energy. The proton affinities (298 K) of HNCCCO, CH3NCCCO, NCCECO, and NCC(CH3)CO are predicted to be 861, 920, 784, and 798 kJ mol(-1), respectively.