Weakly bound molecular clusters are size selected by momentum transfer in a scattering experiment with atoms and then photodissociated by infrared photons to generate spectra as function of their size. From the measured line shifts, which depend sensitively on the excited mode, the cluster size, and the forces involved, detailed information is obtained on the structure of ethylene, methanol, hydrazine, and acetonitrile clusters by accompanying calculations. Double-resonance experiments are employed for identification of different isomers, and their spectra are used as fingerprints for the determination of structural transitions of the methanol hexamer.