The first observation of CH stretching vibrations of isolated aromatic molecular cations in the gas phase was performed by using autoionizaion-detected infrared (ADIR) spectroscopy. Aromatic and alkyl CH stretching vibrations of p-ehylphenol (p-EP) in the cationic ground state were observed and compared with those of the neutral ground state. Distinct high-frequency shifts of these vibrations upon ionization were found. CH stretching vibrations of the p-EP-Ar cluster cation were also measured by infrared photodissociation (IRPD) spectroscopy, roviding essentially the same vibrational spectrum as that of the bare cation.