Morphological changes introduced by chemical etching of Pt, Pt/10–30wt.% Rh, and Rh wires in different oxygen-containing atmospheres at 900–1200°C are studied by scanning electron microscopy and powder X-ray diffraction. The chemical etching ability of different sources for molecular oxygen is ranked as: wet O 2 >N 2 O>dry O 2 >NO. Treatment in these atmospheres gave similar surface patterns with numerous pits and small deposited crystals. Powder X-ray diffraction showed that the surfaces of the tested Rh-containing specimens are more or less covered with Rh 2 O 3 . In wet oxygen and N 2 O atmospheres the formation of Rh 2 O 3 is somewhat suppressed toward lower temperatures and/or higher Rh content. None of the alloys tested at 900°C showed any significant loss of material. Some Rh-containing specimens showed a slight weight gain due to Rh 2 O 3 surface coverage. The occurrence of PtO 2 and RhO 2 in the vapor phase during the etching process was verified by in situ mass spectrometry. Factors of importance for chemical and catalytic etching are discussed.