Infrared spectroscopy was applied to phenol-CO and phenol-CO2 clusters in a molecular beam, and the OH stretching vibrations of these clusters were observed both in their neutral (S-0) and in their cationic (D-0) ground states. Their OH frequency in S-0 shows a small but definite low-frequency shift, indicating that the phenol moiety acts as a proton donor in their hydrogen-bonded structures. The magnitude of the frequency shifts well correlates with proton affinities of the proton-accepting CO and CO2 molecules. The OH stretching vibrations in D-0 showed a substantial low-frequency shift, representing that the hydrogen-bond strength is remarkably enhanced upon ionization. Density functional theoretical calculations of the cluster structures were carried out to support discussion.