The stability of the decagonal phase of Al65Cu15, Co20(d phase) has been studied through thermal annealing, electron beam irradiation and recovery experiments using X-ray and electron diffraction techniques. Long-term annealing of as-cast and melt-spun samples over the temperature 940 to 550°C did not show a decagonal-to-crystalline phase transformation. Upon electron-beam irradiation, the d phase transformed to a f.c.c. phase witha= 351 Å, and the d phase was recovered by heating the f.c.c. phase to 600°C. A simple cubic (s.c.) phase witha= 2.87 Å was found in thin-film samples slightly off the d phase composition, and it transformed partially to the d phase at 600°C. At high temperature (∼900°C), the d and s.c. coexist in the off-composition thin-film and melt-spun samples. Implications of our results on the stability of decagonal Al—Cu—Co are discussed.