Abstract: Both an improved method and apparatus for determining, on an accelerated basis, the susceptibility of a nickel-based alloy to suffer stress corrosion cracking within a steam generator environment is disclosed herein. The method generally comprises the steps of subjecting a mechanically stressed specimen of the alloy to a mixture of steam and hydrogen at a temperature of between about 365.degree. C. to 435.degree. C., wherein the partial steam pressure is between about 10.4 MPa to 30.4 MPa, and the partial hydrogen pressure is between about 32 kPa to 500 kPa. The improved testing apparatus of the invention generally comprises a vessel for containing a stressed specimen and a pressurized atmosphere formed of steam and hydrogen, and an inlet cell for injecting a selected amount of hydrogen into the pressurized atmosphere of the vessel which includes a diffusion membrane which is permeable to hydrogen but impermeable to the remainder of the pressurized atmosphere.