When oxidised at 973 K, annealed 9Cr-lMo steelforms convex oxide ridges at the boundaries of prior austentte grains. In addition to Ms morphological difference these grain boundary oxides show composiiional dissimilarities with the oxides formed within the grains, as suggested by the results of scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX), and substantiated by secondary ion mass spectrometry (SIM)) results. Both EDX and secondary ion imaging confirm the high Si content of the oxides in the ridges. However, it is the synergistic influence of the variations in the chemical composiiion and morphological features of scales that governs the stability of the oxides growing within the alloy grains. These variations have their origin in the oxidaiion process at the alloy grain boundaries. Hence to achieve a more complete understanding of this phenomenon, oxide ridges and the scales within the grains have been extensively characterised by obtaining depth profiles using SIMS and observing the morphological evolution using SEM and surface profilometry. The oxide ridges act as discontinuous interfaces, so that the oxides growing in the adjoining areas experience sufficienlly high stresses to produce spalling. Thefnner the alloy grains, the greater the number of discontinuous interfaces (i.e., grain bouJary ridges) and the stressed area, and therefore the earlier splling occurs.