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)] (1and2) in Pluronic® triblock copolymer P123 core–shell micelles. The spherical nanoparticlesRuMsandOsMs, dispersed in water, were characterized by dynamic light scattering (DLS), cryogenic transmission electron microscopy (cryo-TEM), and synchrotron small-angle X-ray scattering (SAXS; diameterca.15 and 19 nm, respectively). Complexes1and2were highly active towards A2780 human ovarian cancer cells (IC500.17 and 2.50 μM, respectively) and the encapsulated complexes, asRuMsandOsMsnanoparticles, were less potent (IC506.69 μM and 117.5 μM, respectively), but more selective towards cancer cells compared to normal cells.