We describe the electrochemistry of 15 nm diameter silica nanoparticles densely functionalized with ferrocene (FeSiO2) through siloxane couplings. Each nanoparticle bears similar to 600 Fe sites, as measured by potentiometric titration (590 Fe) and diffusion-controlled voltammetry (585 Fe) and estimated by XPS (630 Fe). The nanoparticle ferrocene coverage amounts to ca. a complete monolayer of ferrocene sites, which react electrochemically without mutual interactions and which are apparently fully accessible for diffusion-controlled electrode reactions. Diffusion-controlled voltammetry of the FeSiO2 nanoparticles was observed in dilute methanol dispersions and in more concentrated slurry phases formed in methanol/acetonitrile mixtures. Electrochemical studies reveal interesting behavior in the dilute and more concentrated solutions. Because of the large nanoparticle surface area/volume ratio, the ferrocene-coated silica nanoparticles are capable of storing up to 5 x 10(7) C/m(3) of redox charge its dry phases and 6 x 10(5) C/m(3) in the concentrated slurries.