Infrared reflection absorption spectroscopy of CO has been used as a surface-sensitive probe for the available binding sites of Co-Pd bimetallic particles supported on a thin alumina film. A high-coverage state is obtained on the Co particles in addition to atop sites, attributable to a M(CO)(n) species. Bridge and threefold sites are not detected from CO stretching on the Co particles. When both metals are deposited sequentially at 300 K, Pd easily forms a shell on existing Co particles. In the reverse order, much more Co is required to coat Pd particles, because it nucleates between Pd particles as well as on top of them. For both metals, atop sites are better preserved at various bimetallic compositions because they are statistically less vulnerable than threefold hollow sites. The stretching frequency of CO to a given site is nearly independent of the bimetallic composition. (C) 2003 American Institute of Physics.