Using a refined quantitative TEM technique, we measure the relative mass of individual organometallic rhenium clusters on a graphite support. We observe that whereas 6-Re atom species preserve integrity on precipitation from solution, fragmentation and sintering occur in 7- and 8-Re atoms cluster species. These organometallics are candidates as catalytic materials. The technique is based on high-angle dark-field scanning transmission electron microscope (STEM) imaging. The emphasis in this analysis is on using very high angles of scattering to suppress coherence effects from these clusters and on reliably measuring their relative experimental elastic scattering cross sections.