The ability of Pt/CeO2 to exhibit three-way catalytic activity at low temperatures results from a strong metal-support interaction, which is induced by activating the catalyst under a reducing atmosphere. Carbon monoxide chemisorption measurements show that a loss of exposed metal area occurs during the activation procedure, even though the conditions do not favour platinum sintering. Following activation (and subsequent exposure to air at ambient temperature),the surface ceria is left in a highly reducible state, which is characterised by a subambient peak during temperature-programmed reduction. We envisage that the strong interaction arises according to a traditional SMSI model, by the migration of partially reduced ceria over the surface of the platinum particles (at greater than or equal to 600 degrees C). The resultant high degree of contact between the metal (with high work-function) and the metal oxide (with high band gap) promotes the formation of oxygen vacancies on the ceria surface. Although the presence of this highly reducible ceria-covering blocks conventional three-way sites on the platinum, it provides new sites that are active even at low temperatures. Therefore, unlike most previous explanations of promotion caused by a strong interaction, we propose that the ’support’ becomes the active phase. Re-oxidation at elevated temperatures causes the ceria covering to coalesce, leading initially to its partial thinning, and subsequently to the re-exposure of the platinum particles.