Clusters of thymine were found to undergo facile and extensive photodimerization upon strong UV irradiation. Multiphoton excitation/ionization yields cluster ions with excess energy, which undergo thermal fragmentation during flight in time-of-flight mass spectrometer. Reflectron mass spectrometry was employed to investigate the fragmentation dynamics. Remarkably size-specific modes of fragmentation were found: a cluster ion that consists of an even number of molecules tends to lose two molecules, while one that consists of an odd number of molecules loses one molecule. The strong alternation of intensity in the cluster mass spectrum was found to be due to such distinct fragmentation modes. The two-molecule loss was found to be a single fission process of a dimeric unit, which was in accord with a model of the cluster ion that consists of mostly dimeric units. The culprit was extensive intracluster photodimerization, which was shown to occur through absorption of anomalously large numbers of photons by the cluster system during excitation/ionization.