Spherical polyorganosiloxane nanoparticles containing photoreactive dye labels have been used to study self-diffusion in concentrated colloidal dispersions by forced Rayleigh scattering (FRS). Samples studied contained about 0.5 wt. % of these tracer particles and the order of 50 wt. % nonfunctional host particles dissolved in cyclohexane. To avoid any influence of the label molecules on the particle mobility, the tracers have been equipped with a nonfunctional protective shell. The effect of the thickness of this shell as well as of the label content on particle diffusion has been examined carefully. A much lower tracer mobility has been found for particles with protective shell of thickness less than 1 nm. In some cases, even complementary grating effects have been found in the FRS signals, indicating a different tracer mobility of bleached and nonbleached particles. By diffusion studies using tracers with protective shell sufficiently thick to avoid such unwanted effects, the single particle mobility for polyorganosiloxane microgel dispersions in the highly concentrated regime has been determined. Experimental results show that polyorganosiloxane nanoparticles in organic solvent do not exhibit strong volume swelling as most colloidal model systems but are porous rigid spheres filled with only about 25 wt. % solvent. Therefore, they behave like a very realistic hard sphere system.