The results of electrochemical studies of CO oxidation on clean and bismuth modified Pt(1 1 0)-(1x2) and Pt(1 1 1) surfaces and a supported platinum catalyst are compared. The effect of sub-monolayers of bismuth is to increase the CO oxidation potential on Pt(1 1 0)-(1x2), and decrease it on Pt(1 1 1). This primarily reflects the rather low potential for CO oxidation characteristic of the more open packed Pt(1 1 0)-(1x2) surface, and the higher potential characteristic of the close packed Pt(1 1 1) face. The oxidation potential of CO in the intermixed CO/Bi phases on the single crystals is very similar. The effect of bismuth on the supported platinum catalyst is to increase the CO oxidation potential by a similar amount to that found for Pt(1 1 0)-(1x2). In addition, modification by bismuth results in a concomitant reduction in the CO tolerance of the commercial catalyst. CO oxidation on the catalyst exhibits poisoning by bismuth in a similar fashion to Pt(1 1 0)-(1x2), and the bismuth redox couple on the catalyst is the same as that found on Pt(1 1 0)-(1x2). High coverages of bismuth on the catalyst do not appear to reduce its activity for hydrogen oxidation. These results are discussed in the context of the requirement for surface mediated oxygen transfer during CO electro-oxidation.