The surfaces of a series of PtRh bimetallic catalysts, supported on gamma-Al2O3 (Degussa-C), were characterized by the IR spectrum of pi-bonded ethylene. We have compared results for ethylene adsorbed on 3% Pt/Al2O3, 1.6% Rh/Al2O3, nonalloy PtRh/Al2O3, and alloy PtRh/Al2O3 catalysts. We find that the C=C stretch frequency can reveal the presence of PtRh alloy particles. The C=C stretch frequency increases by approximate to 60-80 cm(-1) for pi-bonded ethylene adsorbed on PtRh alloy sites compared with pi-bonded ethylene adsorbed on monometallic Pt or Ph particles. The definitive correlation of pi-bonded ethylene bands with PtRh alloy particles was achieved by using X-ray diffraction, scanning transmission electron microscopy, and analytical electron microscopy to classify the bimetallic catalysts as alloy or nonalloy. Once established, the correlation permits detection of alloy particles that are too small to be characterized by noninfrared techniques.