A new experimental technique, based on high resolution time of flight analysis of ions, has demonstrated that one photon ionization of dimers, trimers, and tetramers, is accompanied by dissociation of the cluster ion to form either clusters of lower order, new ion structures, or other products formed by intra-cluster ion molecule reactions. The method is based measuring the inevitable kinetic energy released in the dissociation process. By maintaining a low electric field across the ionization region, and optimizing the instrumental parameters, it is possible to measure kinetic energy releases as low as 2 meV. Examples of systems investigated include clusters of acetylene, ethylene, NO, CH3OH, CH3SH, CH3Cl, and C2H5Cl. The results showed that even at the lowest photon energies used, ionization without dissociation consisted of at most 5% of the total signal. In some ions, such as C2H2 clusters, 100% of the one photon ionization process is via dissociation.