A newly developed technique for infrared spectroscopy of cold molecular cations in the gas phase is reviewed. Very high Rydberg states converging to the first ionization threshold are prepared by two-color double resonance excitation. Vibrational exictation of the ion core of the Rydberg states induces autoionization, leading to the molecular ion. An infrared spectrum is obtained by monitoring the ion current due to the autoionization as a function of the vibrational excitation laser frequency. The observed spectrum practically provides the vibrational frequency of the bare ion. This new technique is called autoionization-detected infrared (ADIR) spectroscopy, and it is very advantageous to observe high frequency vibrations of molecular cations, such as OH and CH stretches. The following applications of ADIR spectroscopy to the phenol derivative cations are reviewed in this paper; (1) OH stretching vibrations of typical intramolecular hydrogen-bonded cations. (2) Unconventional intramolecular hydrogen bonds between the hydroxyl and alkyl groups. (3) Aromatic and alkyl CH stretching vibrations in aromatic cations.