Silica-supported Pt/SnOx catalysts used for low-temperature CO oxidation have been prepared without and with an Fe promoter. Reaction studies demonstrate that the addition of the Fe promoter results in higher catalytic activity in the presence of 8 at% CO2 and a lower decay rate. Ion scattering spectroscopy (ISS) has been used to examine the outermost atomic layers of the promoted and nonpromoted catalysts before and after activation by a reductive pretreatment. The nonpromoted catalyst exhibits agglomeration of the platinized tin oxide film exposing the catalytically inactive silica support. This agglomeration does not occur when Fe is present, and a large catalytically active surface area is maintained during the reduction.