Preliminary results are reported of the observation of an intermetallic phase, formed during the oxidation of a 9 wt% Cr-1 wt% Mo steel, the electron diffraction patterns from which exhibit 5 symmetry. The Mo, Fe-rich phase, of approximate composition 37wt% Mo-45wt% Fe-11wt% Cr and 7wt% Si, forms 150μm beneath the oxide scale when the steel is exposed to an atmosphere bf high-pressure CO2at a temperature of 560°C. High-resolution electron microscopy has revealed that the structure of the Mo,Fe-rich phase consists of an intergrowth of microdomains of several phases, which may be interpreted as pentagonal tetrahedrally close-packed (t.c.p.) phases.