Abstract Duplex type aluminum bronzes usually exhibit a structure consisting of α and the interconnecting eutectoid matrix of α and γ particles. These γ particles are highly anodic to the α particles. Usually the β phase is retained because it is less anodic to α particles than the γ phase. In our previous study, observation of the development of dealuminification corrosion of Cu–9Al–2Mn alloy in an early stage was first carried out by in situ scanning tunneling microscope (STM). It has been indicated that the existence of the β phase has a negative effect on dealuminification corrosion of aluminum bronzes, which occurs preferentially on the β phase. In order to determine the influence of β phase transformation in Cu–Al–Mn alloys on their dealuminification micro-mechanism, the change in the microstructure before and after dealuminification corrosion of Cu–9Al–2Mn alloy was investigated in the present work. Results indicate that dealuminification corrosion of Cu–9Al–2Mn alloy is associated with martensite transformation of metastable β 1 phase and occurs within the β′ 1 martensite plates, in which there is a large amount of stacking faults. Micro-pits and fine corrosion products were also observed by TEM. The reason why β 1' martensite plates suffered a preferential dealuminification corrosion is martensite transformation of β-Cu 3Al phase and the higher density of stacking faults in β 1' martensite plates.