The infrared spectra of the above compounds, and the Raman spectra of the parent compound and its 2,3,5,6-D4, 1-D, and 1-Me derivatives have been determined. The effects of electron-withdrawing infstituents, N-methylation, and deuteration on the two intense infrared bands in the 1480–1650 cm−1 region are not compatible with one being a C=O and the other a C=C stretching band. These two vibrations must be extensively mixed; occasionally very pronounced medium effects, which are not straightforward, are observed for one or both of these bands. The spectra are not in accord with a mostly zwitterionic (N-protonated pyrid-4-oxide) structure (II) of 4-pyridone. Band assignments are made for the parent compound.