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Bend tests were made using sodium chloride single crystals which had previously been exposed to one of several atmospheres at room temperature. Crystals exposed to moist carbon dioxide showed the most consistent brittle behavior while those aged in moist air containing hydrogen chloride showed more ductile behavior. Evidence is given which indicates that hydroxide ion is needed for carbon dioxide to cause embrittlement. Furthermore, room-temperature hydrolysis is shown by experiments in which the increase of alkalinity of finely divided sodium chloride is measured upon long exposure to moist air. These results lead to the proposal of a new mechanism to explain age embrittlement of sodium chloride crystals: sodium chloride crystals containing hydroxide ion react with carbon dioxide to become embrittled. Crystal surfaces initially free from hydroxide can become contaminated with it by reaction with water vapor of the air. Changes in ductility due to physical adsorption of carbon dioxide or hydrogen chloride were not observed.