The formation of quasi-crystalline PtRh two dimensional films supported on Pt{100} is described. For the first time, the voltammetry of PtRh{100} single crystal alloys covering the whole range of alloy composition is reported. Synthesis follows a similar procedure to that described previously for the formation of PtPd alloys supported on Pt{hkl} but with some important provisos concerning the final annealing step. CO electrooxidation was used as a probe reaction and for certain PtRh{100} surface alloys, unusually high electrocatalytic activity was observed relative to monometallic Pt and Rh electrodes. The flame annealing of a "PtRhPt sandwich" precursor structure was found to be the best method of forming the PtRh alloy surfaces. For PtRh films annealed under nitrogen, significant phase separation was observed in agreement with previous surface science studies of PtRh adlayers on Pt{100} annealed in the absence of oxygen. In addition, an excess of Rh in the "sandwich structure" from which the alloy was formed tended to preclude good alloy formation. It is suggested that the protocols for thin film formation described may prove useful in many other important electrocatalytic systems. (C) 2008 Elsevier B.V. All rights reserved.