The stress-corrosion properties of magnesium-5% aluminium alloy in distilled water and other solutions were investigated. Stress corrosion tests were also performed on other commercial magnesium-base alloys and on high-purity magnesium. Rapid transcrystalline cracking of stressed magnesium-5% aluminium alloy in distilled water occurred, following neither grain boundaries nor twin planes, if the alloy had a precipitated second phase or not. Stress-corrosion was associated with a localized penetrative transcrystalline attack, up to 2-3 grains deep. Stress-corrosion resistance decreased with increase in iron content and plastic deformation, but was improved by annealing at 200C after deformation. Dissolved oxygen enabled stress-corrosion to operate. Trans-crystalline stress-corrosion fractures occurred in saturated magnesium carbonate, 0.5% potassium fluoride and other solutions. The commerical alloys containing aluminium were less resistant. Results show stress-corrosion cracking in magnesium-base alloys to be partly electrochemical. Cracking may be associated with minor impurities, e. g. iron, which negatively effect ordinary corrosion resistance.