The concentration of alkali metal ions caused by the movement of moisture in concrete has been examined by chemical analysis and X-ray microprobe analysis. It has been found that there is a significant increase in the concentration of these ions close to the surface from which moisture is evaporating with local very high concentrations which have been shown to be due to the crystallisation of alkali sulphates. The significance of this effect in promoting alkali aggregate reaction is discussed. © 1979.