Abstract The phase transformations in the Cu–35 at.%Mn–25 at.%Al alloy have been investigated by means of transmission electron microscopy (TEM) and energy-dispersive X-ray spectrometry (EDS). In as-quenched condition, the microstructure of the alloy was a mixture of (L2 1 + B2 + L-J) phases. This is different from that observed by previous workers in the Cu 3− X Mn X Al alloys with X ≤ 1.0. When the as-quenched alloy was aged at 460 °C for short times, γ-brass precipitates started to occur at anti-phase boundaries (APBs). After prolonged aging at 460 °C, the γ-brass precipitates grew and β-Mn precipitates were formed at the regions contiguous to the γ-brass precipitates. The coexistence of (γ-brass + β-Mn) has never been observed by previous workers in Cu–Mn–Al alloy systems before.