The mechanism of intergranular stress corrosion cracking in 70 Cu–30 Zn alloy was investigated by means of electron microscopy, electron diffraction and stress corrosion cracking tests using Mattsson’s solutions (0.05 M/L CuSO4+0.5M/L (NH4)2SO4) at pH values of 2.0, 7.4 and 10.0. It was found from the stress corrosion cracking tests that, at pH 7.4 a Cu2O surface film was formed on the brass and intergranular cracks occurred easily, but at pH values of 2.0 and 10.0, the brass was unsusceptible to cracking. The observation of bulk specimens by a replica technique indicated that at pH 7.4 micro-attacks initially nucleated within grains but two hours later corrosion was remarkably promoted in grain boundaries. Although the corrosion behaviours of bulk specimens depended considerably on pH values, the mode of micro-attack nucleation in thin-foil specimens did not change with pH values, but the preferential micro-dissolution in the slip steps was predominant. At pH 7.4 micro-pits formed in the slip steps consisted of Cu2O and Cu layers which were cathodic to the brass substrate, so that the brass surface was covered with these layers. As the Cu2O and Cu layers would lock the movement of dislocations on emergent slip planes within grains the absorption of dislocations and vacancies in grain boundaries would increase. In consequence, the chemical reaction in the grain boundaries would be accelerated than that in the case of no surface film layers, and the path of cracks would be formed in the grain boundaries.