The treatment of polystyrene and poly(styrene-acrylonitrile) with ozone allows the examination of the effect of a mildly oxidized surface on the activity of electroless catalysts. Ozonation, compared to photo-oxidation, produces a lightly oxidized surface that inhibits, but does not deactivate, electroless catalysts. The treated polymer surfaces were examined by x-ray photoelectron spectroscopy (XPS) at several electron emission angles. By varying the electron emission angle, both the amount and the depth distribution of ozonation products can be determined in the near surface region. XPS detected less than a monolayer of oxidation on the ozonated surfaces accompanied by the accumulation of about 0.5 monolayer of aromatic hydrocarbon. Even though total catalyst poisoning does not occur, the fact that catalyst inhibition occurs on a marginally altered surface has implications concerning treatments of polymer surfaces before electroless deposition.