We report a systematic investigation on the structural and electronic effects of carbon-supported Pt(x)Pd(1-x) bimetallic nanoparticles on the oxygen reduction reaction (ORR) and methanol oxidation reaction (MOR) in acid electrolyte. Pt(x)Pd(1-x)/C nanocatalysts with various Pt/Pd atomic ratios (x=0.25, 0.5, and 0.75) were synthesized by using a borohydride-reduction method. Rotating-disk electrode measurements revealed that the Pt(3)Pd(1)/C nanocatalyst has a synergistic effect on the ORR, showing 50% enhancement, and an antagonistic effect on the MOR, showing 90% reduction, relative to JM 20 Pt/C on a mass basis. The extent of alloying and Pt d-band vacancies of the Pt(x)Pd(1-x)/C nanocatalysts were explored by extended X-ray absorption fine-structure spectroscopy (EXAFS) and X-ray absorption near-edge structure spectroscopy (XANES). The structure-activity relationship indicates that ORR activity and methanol tolerance of the nanocatalysts strongly depend on their extent of alloying and d-band vacancies. The optimal composition for enhanced ORR activity is Pt(3)Pd(1)/C, with high extent of alloying and low Pt d-band vacancies, owing to favorable O-O scission and inhibited formation of oxygenated intermediates. MOR activity also shows structure dependence. For example, Pt(1)Pd(3)/C with Pt(rich-core)Pd(rich-shell) structure possesses lower MOR activity than the Pt(3)Pd(1)/C nanocatalyst with random alloy structure. Herein, extent of alloying and d-band vacancies reveal new insights into the synergistic and antagonistic effects of the Pt(x)Pd(1-x)/C nanocatalysts on surface reactivity.