A new method for infrared spectroscopy of cold molecular ions is proposed, and its application to the phenol ion is presented. Very high Rydberg states converging to the first ionization threshold are prepared by two-color double-resonance excitation. Vibrational excitation 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 as a function of the vibrational excitation laser frequency. The observed spectrum practically provides the vibrational frequency of the bare ion.