Understanding the evolution of the composition and atomic structure of nanoalloy catalysts in the ethanol oxidation reaction (EOR) is essential for the design of active and robust catalysts for direct ethanol fuel cells. This article describes a study of carbon-supported platinum-ruthenium electrocatalysts (PtRu/C) with different bimetallic compositions and their activities in the EOR, an important anode reaction in direct ethanol fuel cells (DEFCs). The study focused on establishing the relationship between the catalyst's composition, atomic structure, and catalytic activity for the EOR. Ex situ and in situ synchrotron high-energy X-ray diffraction (HE-XRD) experiments coupled with atomic pair distribution function (PDF) analysis and in situ energy-dispersive X-ray (EDX) analysis were employed to probe the composition and structural evolution of the catalysts during the in situ EOR inside a membrane electrode assembly (MEA) in the fuel cell. The results revealed an intriguing composition-structure-activity relationship for the PtRu electrocatalysts under EOR experimental conditions. In particular, the alloy with a Pt/Ru ratio of ∼50:50 was found to exhibit a maximum EOR activity as a function of the bimetallic composition. This composition-activity relationship coincides with the relationship between the Pt interatomic distances and coordination numbers and the bimetallic composition. Notably, the catalytic activities of the PtRu electrocatalysts showed a significant improvement during the EOR, which can be related to atomic-level structural changes in the nanoalloys occurring during the EOR, as indicated by in situ HE-XRD/PDF/EDX data. The findings shed some new light on the mechanism of the ethanol oxidation reaction over bimetallic alloy nanocatalysts, which is important for the rational design and synthesis of active nanoalloy catalysts for DEFCs. (Graph Presented).