A phosphorus-modified electrolytic silver catalyst was prepared and used as catalyst in the oxidative dehydrogenation of glycol to glyoxal. The yield of glyoxal was observed as high as 82% at 98% conversion for the Ag-P catalyst, while 62% at 89% conversion for the pure electrolytic silver. The formation of the surface compounds between the phosphorus additives and the silver surface was demonstrated by means of XPS and SEM. It caused the decrease of the surface concentration of atomic oxygen species, and restrained the decomposition and total oxidation of adsorbed glycol to C-1 products.