Intramolecular proton transfers in the cationic and anionic states of o-hydroxybenzaldehyde derivatives were investigated by using electron spin resonance (ESR) spectroscopy and computational chemistry. The hyperfine structures observed in the ESR spectra for these states were analyzed. From a comparison between the experimental and computational results, it is considered that these states are not susceptible to intramolecular proton transfer, which is consistent with the nodal patterns of the wave functions.