Two 9Cr–1Mo steels containing 0·26 and 0·75 wt-%Si were subjected to different annealing treatments to vary the prior austenite grain size. When oxidised in air at 973 K, the grain boundaries oxidise at higher rates than the areas within the grains, and thus develop oxide ridges. In addition to this morphological difference these grain boundary ridges also show compositional dissimilarities from the oxides formed within the grains, as suggested by the results from analyses using scanning electron microscopy and secondary ion mass spectrometry (SIMS) at these locations. Both energy dispersive X-ray analysis and secondary ion imaging confirm the high Si content of the oxides in the ridges. Oxide ridges and the scales within the grains of the oxidised alloys having different grain sizes and Si contents have been extensively characterised by obtaining depth profiles for different elements using SIMS. An important observation is that there is a synergistic influence of the grain size and the Si content of the alloy that governs the chemical composition and morphological features of scales, which in turn governs the stability of the oxides growing within the alloy grains. As suggested by thermogravimetry, scales growing over the alloy having low Si content and fine grain size show the lowest stability and earliest onset of spalling, whereas the scale on the steel having high Si content and large grain size is most resistant to spalling.