Synopsis The possible effect of sodium chloride in increasing the alkalinity of concrete and exacerbating alkali-aggregate reaction has been investigated by experiments involving the addition of sodium chloride and synthetic seawater to cement pastes, mortars and concretes. Measurements of the composition of the pore solution of cement pastes and expansion of mortars and concretes made with reactive aggregates are reported. It was found that small additions of both sodium chloride and synthetic seawater elevated the hydroxyI ion concentration of the cement pastes to a level similar to that produced by a Portland cement with an equivalent alkali level. Similarly the sodium chloride and synthetic seawater had an effect on the expansion of mortars and concretes which was in line with the effect produced by an equivalent amount of alkali in the cement.