Infrared (IR) spectroscopy of mass-selected neutral molecular clusters in supersonic beams is newly developed by combining a vacuum ultraviolet (VUV) photoionization. In this spectroscopy, the ion intensity of the mass-selected species is used as a measure of the population of the corresponding neutral cluster species. The IR excitation of the neutral species may cause the vibrational predissociation leading to the population reduction, which appears as a dip spectrum of the photoionization signal. This method can be called VUV-ionization detected IR predissociation spectroscopy (VUV-ID-IRPDS). The method is applied to the IR spectra of ammonia dimer and trimer. (c) 2006 Elsevier B.V. All rights reserved.