A coaxial infrared laser-ion beam photoabsorption spectrometer was constructed for the measurement of the vibrational spectrum of an acrylonitrile cluster cation, (CH2CHCN)(n)(+) (n = 3-10), by detecting photodissociation products of the cluster ion in the wavenumber range of 925-1090 cm(-1). All the spectra measured in this n-range have an intense peak at 970 cm(-1), which is associated with a C=C-H out-of-plane vibration of an acrylonitrile molecule in the cluster ion. The comparison of the spectra with an ab initio calculation shows that the cluster ion consists of a dimer ion core solvated with neutral acrylonitrile molecules. On the other hand, the acrylonitrile cluster anions did not exhibit any optical absorption in the wavenumber range studied. This finding is consistent with our previous result that intracluster polymerization takes place when an electron is attached onto a neutral cluster of acrylonitrile.