Reaction fronts formed during the O2 + H-2 and NO + H-2 reactions on a 70 Angstrom thick microstructured Rh film on a Pt(100) substrate have been investigated using photoemission electron microscopy (PEEM) and scanning photoelectron microscopy (SPEM). The reactions were carried out in the 10(-7) mbar range at T = 670 and 700 K. The photoelectron spectra revealed that the thin Rh film was alloyed with Pt due to vertical intermixing with the underlying Pt(100) substrate. The surface Rh/Pt ratio was found to vary from similar to 4 for an oxygen-free film to 4.8 in oxygen ambient. The O 1s spectra indicated the presence of oxide species along with chemisorbed oxygen on the alloyed film. The chemical maps revealed an enhanced tendency for oxide formation at the boundaries of the Rh/Pt film leading, under reducing conditions, to residual oxide and Ph enrichment expanding laterally to similar to 10 mu m inside the film. This composition inhomogeneity can explain the peculiar ring patterns near Pt/Rh interfaces that were observed by PEEM under the reaction conditions. (C) 1999 Elsevier Science B.V. All rights reserved.