The behavior of ultra- thin (<1 ML) films of vanadium oxide on Rh( 111) during the H-2 + O-2 reaction was studied in the 10(-6)-10(-4) mbar range with photoemission electron microscopy (PEEM). One observes that under reaction conditions the V-oxide condenses forming a stripe pattern of macroscopic dimensions on the surface. The pattern is characterized by an intrinsic wave length which obeys a power law dependence on the reactants pressure. The origin of the pattern is discussed within the concept of reactive phase separation, i.e. the strongly attractive interactions between V and O and/or changes in the interfacial energies caused by reduction are presumably the main driving forces for condensation.