Exchange of [Rh(NH 3 ) 5 (H 2 O)] 3 + ions from aqueous solution into NaY starts at the surface of the zeolite grains; ion penetration into subsurface cavities is slow. A marked rhodium concentration profile from the surface to the interior of the granuli vanishes only after an exchange time of three days, as evidenced by X-ray photoelectron spectroscopy (XPS). Heating the [Rh(NH 3 ) 5 (H 2 O)] 3 + -loaded NaY in argon up to 500°C leads to 100% autoreduction of the rhodium and formation of rather large rhodium particles. Heating the same precursor in 1 bar of O 2 up to 380°C yields a mixture of the oxides RhO 2 and Rh 2 O 3 and the ions Rh 3 + and Rh + . The oxides and Rh 3 + ions are located in the supercages, while Rh + is most likely in the small cages. After calcination to 500°C, Rh 2 O 3 is the only oxide present; some of the Rh 3 + ions have migrated into sodalite cages and hexagonal prisms. Reaction of Rh 2 O 3 with zeolite protons produces more Rh 3 + ions; a maximum concentration is achieved with HY when heated to 500°C. Reduction of the calcined samples in flowing H 2 produces small rhodium particles located primarily inside the zeolite supercages. The extent of this reduction depends on the proton concentration and the temperature because the equilibrium between Rh 0 , protons, and Rh + prevents 100% formation of Rh 0 when the H + concentration is appreciable; in HY a Rh 0 + ratio 1 is found. As a consequence of strong proton anchoring, the rhodium particle size in HY remains < 1 nm after H 2 reduction. Formation of rhodium-proton adducts lowers the propensity of rhodium to adsorb H 2 at low temperature.