We have investigated, using temperature-programed desorption, high-resolution electron energy loss spectroscopy and low-energy electron diffraction, the formation and decomposition of the suboxides of Ru which form after exposure of NO2 to the Ru(001) surface. The NO2 (radical) molecule is not limited by any significant kinetic barrier to its decomposition even at the high oxygen coverages considered. It is this barrier which restricts measurable rates of dissociative chemisorption of O-2 to surface coverages below 0.5 under ultrahigh vacuum conditions. We have identified a subsurface state which is occupied after saturation of the surface overlayer. Utilizing isotopically labeled molecular oxygen and a detailed mixing/segregation model, we have been able to elucidate and quantify the nature of mixing between the surface and subsurface oxygen as a function of total oxygen uptake.