The stress corrosion cracking behaviour of two nickel base superalloys (A718 and A690) is investigated in aerated supercritical water by means of constant extension rate tensile (CERT) tests at 400°C and 25 MPa. Alloy 718 is observed to be extremely sensitive to intergranular stress corrosion cracking (IGSCC) while alloy 690 is found to be immune to IGSCC. CERT tests are also carried out in air and under vacuum on both alloys at 600°C, i.e. in a temperature domain where nickel base superalloys exhibit oxygen embrittlement. The results obtained strongly suggest that oxidation is controlling both the stress corrosion crack initiation and propagation stages of nickel base superalloys in aerated supercritical water. In the light of these results, the main issue of material selection for the construction of supercritical water oxidation (SCWO) reactors is discussed.