The use of beryllia as the basis of an all-ceramic fuel element for high-temperature reactors cooled by ambient air depends on inhibition of corrosion of beryllium oxide by protecting the surface of the fuel element. Alumina is a promising coating material. The development of corrosion resistant alumina coatings for BeO spheres is described, and results of corrosion, accelerated corrosion and neutron irradiation tests are presented. The limitations of the coating are discussed in detail Porous coatings can fail by 'undermining', but dense coatings are probably satisfactory for at least two years' operation out-of-pile at 1200ºC and for much longer times at lower temperatures. Satisfactory operation at 1200ºC for one year has been proved in a long term test. The eventual failure mechanism will probably be associated with the growth of a two layer reaction zone. Neutron irradiation tests indicate that a dose of 1020 nvt and possibly 1.6—2 x 102º nvt should be acceptable for retention of coating-to-BeO bond during simultaneous exposure above 500-700ºC to fast neutrons and moist air.