The steamside oxide formed on two TP347H superheater tubes was compared. The two specimens investigated were exposed in situ in power plants in Denmark, one specimen was coarse,grained and the other was fine-grained. Parts of both the coarse-grained and fine-grained specimens were turned (machined on the inner side) to give a constant metal thickness so more precise wall thickness measurements could be undertaken. Machined and non-machined areas were investigated using light optical microscopy, and scanning electron microscopy with EDS analysis. The oxide on the fine-grained specimen was also investigated with grazing incidence X-ray diffraction. Results from coarse-rained and fine-grained specimens (machined and non-machined) show that grain size influenced oxide thickness and morphology. The oxides from non-machined specimens had an outer iron rich oxide and an inner iron chromium oxide. However a thinner oxide had grown on the fine-grained steel. The machining of fine-grained and coarse-grained specimens resulted in a thin chromium rich oxide layer. The presence of surface deformation of the inner metal surface was evident on the coarse-grained specimen but not on the fine-grained specimen. However other indications that the fine-grained specimen had been deformed were observed. The influence of temperature, grain size, surface finish and exposure duration is discussed.