Yes / We report the encapsulation of highly hydrophobic 16-electron organometallic ruthenium and osmium carborane complexes [Ru/Os(p-cymene)(1,2-dicarba-closo-dodecarborane-1,2-dithiolate)] (1 and 2) in Pluronic® triblock copolymer P123 core–shell micelles. The spherical nanoparticles RuMs and OsMs, dispersed in water, were characterized by dynamic light scattering (DLS), cryogenic transmission electron microscopy (cryo-TEM), and synchrotron small-angle X-ray scattering (SAXS; diameter ca. 15 and 19 nm, respectively). Complexes 1 and 2 were highly active towards A2780 human ovarian cancer cells (IC50 0.17 and 2.50 μM, respectively) and the encapsulated complexes, as RuMs and OsMs nanoparticles, were less potent (IC50 6.69 μM and 117.5 μM, respectively), but more selective towards cancer cells compared to normal cells. / We thank the Leverhulme Trust (Early Career Fellowship no. ECF-2013-414 to NPEB), the University of Warwick (Grant no. RDF 2013-14 to NPEB), the Swiss National Science Foundation (Grant no. PA00P2_145308 to NPEB and PBNEP2_142949 to APB), the ERC (Grant no. 247450 to PJS), EPSRC (EP/G004897/ 1 to APB, and EP/F034210/1 to PJS), Institute of Advanced Study (IAS) – University of Warwick (Fellowship to JJSB), and Science City (AWM/ERDF) for support. We thank the Wellcome Trust (055663/Z/98/Z) for funding to the Electron Microscopy Facility, School of Life Sciences, University of Warwick.