The effect of the subsurface in monomer-dimer (CO-O-2) surface reaction is studied by Monte Carlo simulation. The presence of the subsurface adds some extra reaction steps to the usual three. It is shown that the presence of the subsurface favours the enhancement of the steady reactive state by reducing appreciably the poisoning of the surface by the adsorbed dimer atoms. Under certain conditions the usual second order phase transition in CO-O-2 reaction can be eliminated. The role played by the two way diffusion of adsorbed dimer atoms between surface and subsurface in the variation of steady reactive state is clearly illustrated.