Au (x) Pd1-x (x = 0, 0.25, 0.5, 0.75, 1) nanoparticle (NP) catalysts (8-11 nm) were synthesized by a one-pot reaction strategy using colloidal chemistry. XPS depth profiles with variable X-ray energies and scanning transmission electron microscopy (STEM) analyses show that the as-synthesized Au (x) Pd1-x (x = 0.25 and 0.5) bimetallic NPs have gradient alloy structures with Au-rich cores and Pd-rich shells. The evolution of composition and structure in the surface region corresponding to a mean free path of 0.6-0.8 nm (i.e., 2-3 layers to the bulk from the particle surface) was studied with ambient pressure X-ray photoelectron spectroscopy (AP-XPS) under CO/O-2 reaction in the Torr pressure regime. Under the reaction conditions of 80 mTorr CO and 200 mTorr O-2 at 200 A degrees C, the surface region of Au0.75Pd0.25 NP is Au-rich (similar to 70% by Au). All Au (x) Pd1-x (x = 0.25, 0.5, 0.75) NP catalysts have higher turnover rates for the model CO/O-2 reaction than pure Pd and pure Au NPs. The Pd-rich Au0.25Pd0.75 NPs show the highest turnover rates and the Pd-rich Au0.5Pd0.5 NPs the lowest turnover rates at 200 A degrees C. Interestingly, the Au-rich Au0.75Pd0.25 NPs exhibit steady-state turnover rates which are intermediate to those of the Pd-rich bimetallic nanoparticles.