A new type of ion implantation technique is used to create a non-equilibrium Pt-Sn(IMP) near-surface alloy with ca. 8.6 at% Sn. The surface composition was determined by low-energy ion-scattering (LEIS). The kinetics of the electrooxidation of CO and 2% CO/H-2 mixtures on Pt-Sn(IMP) is essentially identical to that of Pt3Sn(110). The fact that any non-equilibrium composition can be easily prepared by this implantation method opens an interesting practical opportunity to create a new Pt-Sn alloy fuel cell catalyst having an otherwise unobtainable surface composition of Sn. This method also appears to have general utility in alloy catalysis as a means of exploring compositions in thermodynamically unfavorable regions of the bulk phase diagram.