Environmentally-assisted cracking is studied for an Al-Zn-Mg alloy in different gaseous atmospheres. A sophisticated experimental installation with an ultra-high-vacuum chamber (10-5) is used to maintain and monitor a high-purity atmosphere surrounding the specimen. The experimental data show that water vapor is the main factor responsible for the enhancement of the crack growth rates as compared with those in the high vacuum. Embrittlement is attributed to the influence of hydrogen produced as a result of the surface reaction of water vapor with the fresh metal surface near the crack tip.