The specimens of O-phase Ti-22Al-25Nb (at%) intermetallics coated with silica-based enamel received mass gains of about 1 mg/cm2, after 300 h of oxidation or hot corrosion at 800 °C. These rates were much faster than the growth rates of silica films at the same temperature. To understand this phenomena, the specimens were analyzed using SEM, XRD, EPMA and TEM. An oxide layer with thickness of several μm was observed at the enamel/substrate interface of the coated specimens after either oxidation or hot corrosion. XRD and TEM analysis revealed the newly formed oxide layer was composed of α-Al2O3, Al2SiO5, Al2TiO5, rutile-TiO2 and NbO2. It was shown by EPMA profiling that an Al-depleted zone was located just beneath the oxides. It was proposed that the solid reactions between the enamel coating and the O-phase Ti-Al-Nb played important roles for the oxidation and hot corrosion behavior of the coated specimens.