We demonstrate that a capillary-type threshold photoelectron analyzer capable of an energy resolution of ±1-2 meV in two-color (1+1′) resonance ionization is a promising development in gas-phase cation vibrational spectroscopy, especially for studying low-frequency vibrational modes. The first example studied here is toluene. Several observed photoelectron vibrational bands due to the toluene cation show good correlation with our ab initio calculations. The second example studied is n-propylbenzene. The rotational isomers trans and gauche of the n-propylbenzene cat ion are clearly distinguished in our threshold photoelectron measurements. The trans and gauche isomers of this cation show quite different vibrational spectra in the low frequency region.