The vibrational features of p-chlorobenzonitrile in its first electronically excited state S₁ and cationic ground state D₀ have been investigated by two-color resonance enhanced two-photon ionization and mass analyzed threshold ionization spectroscopy. The excitation energy of S₁ ← S₀ and the ionization energy of ³⁵Cl and ³⁷Cl isotopomers of p-chlorobenzonitrile are determined to be 35818 ± 2, and 76846 ± 5 cm^–1, respectively. These two isotopomers have similar vibrational features. Most of the active vibrations in the S₁ and D₀ states are related to the motions of the in-plane ring deformation. The stable structures and vibrational frequencies of p-chlorobenzonitrile are also calculated by the B3LYP/aug-cc-pVDZ method for the S₀ and D₀ states, and TD-B3LYP/aug-cc-pVDZ method for the S₁ state. The changes in the molecular geometry are discussed in the S₁ ← S₀ photoexcitation process and the D₀ ← S₁ photoionization process. The comparisons between the transition energy of p-chlorophenol, p-chloroaniline, p-chloroanisole, and p-chlorobenzonitrile with those of phenol, anisole, aniline, and benzonitrile provide an insight into the substitution effect of Cl atom.