This paper reports on the measurement of the corrosion rate, and the stress corrosion cracking threshold stress, for ultra-high-purity Mg-5Zn. The corrosion rate was higher than the intrinsic corrosion rate of ultra-high-purity Mg, attributed to the quality of the corrosion product film. The threshold stress for stress corrosion cracking, at an applied stress rate of 0.00016 MPa s -1 , was equal to 0.7 times the yield stress in air. The ductility of the cracking indicated that the stress corrosion cracking mechanism was probably hydrogen enhanced localized plasticity.