An experimental method is presented for assigning vibronic spectra of van der Waals molecules based on the measurement of the velocity-and mass-resolved resonance enhanced multiphoton ionisation (VMR-REMPI) spectrum. The basis of the technique is that at each wavelength one measures the velocity map image of the ions produced at a chosen mass. If this mass corresponds to the parent ion for the transition, the image shows a central spot only. However, if this mass is from a fragment ion, the image is enlarged due to the velocity gained by the ion during dissociation. VMR-REMPI spectra consist of plots of the integrated signal in an appropriate image area versus laser wavelength or frequency. The technique is demonstrated with two examples, benzene-acetylene, (n = 1-3) and p-difluorobenzene-Ar. Application of the technique to the former system leads to a reassignment of the spectrum. (C) 2003 Elsevier Science B.V. All rights reserved.