The microstructures produced by the heat treatment of a commercial age hardening At-4.4 Cu alloy have been examined by thin foil electron microscopy. Whilst there is similarity of the microstructures in the commercial alloy to those which have been reported for simple binary At-Cu alloys, there is a strong association of dislocation structures and incoherent precipitates with undissolved Mn bearing intermetallic particles. Fatigue tests made on the alloy at both 50 Hz and 20 kHz have shown that there are changes in microstructure during fatigue; more markedly at 20 kHz in which tests the heating effect has some importance. There is some evidence of both accelerated ageing and also the by-passing or resolution of coherent phases during fatigue, particular at 20 kHz.