A large number of studies are devoted to the investigation of the biomolecular ionization and fragmentation dynamics underlying biological radiation damage. Most of these studies have been based on gas‐phase collisions with isolated DNA building blocks. The radiobiological significance of these studies is often questioned because of the lack of a chemical environment. To clarify this aspect, we studied interactions of keV ions with isolated nucleobases and with nucleobase clusters by means of coincidence time‐of‐flight spectrometry. Significant changes already show up in the molecular fragmentation patterns of very small clusters.