When Pt silicides are formed in an ambient containing oxygen, or using Pt films containing an appreciable amount of oxygen, the formation rates of the silicides are reduced. In the latter case the reduced rates are attributed to the formation of an oxide layer, from the oxygen contained, between the surface Pt and the Pt2Si and PtSi phases. To remove such difficulties, we use Pt films containing little oxygen, and study the effect of oxygen on the formation of both Pt silicides in an oxygen ambient. It is shown that oxygen diffuses from the ambient into Pt during the stage of Pt2Si formation, and an oxide layer is formed under the surface when the front of the Pt2Si phase meets the oxygen in the film, which hinders further growth of the Pt2Si phase. Subsequent formation of the PtSi phase proceeds until it meets the oxide layer and is hindered from further growth. Accordingly, the oxide layer described has little effect on the bulk growth of these two silicides, and other mechanisms are needed to account for the reduced rates by oxygen for such systems, such as that described by the surface potential model.