LEED studies of the co-adsorption of hydrogen and oxygen on tungsten (001) have revealed the formation of distinct surface phases. At room temperature and a total coverage of about one-half monolayer the adsorption is competitive: oxygen and hydrogen produce separate domains on the surface. When the crystal is heated, a homogeneous, mixed phase is produced. The mixing is reversible and no hysteresis is observed upon cooling. The transition occurs at a temperature substantially below the disordering temperature for the pure oxygen structure; it is ascribed to an effective repulsion between adsorbed H and O. Studies of the adsorption rate show that in this coverage range, the hydrogen sticking probability is independent of any preadsorbed oxygen.