Recent site and lab investigations carried out by BRE on problems of sulfate attack have shown that the mineral thaumasite has been responsible for the deterioration of concretes and mortars specifically designed to give good sulfate resistance. Previous work indicated that the use of blast furnace slag cement could improve the resistance of limestone aggregate concretes. The results of further work presented in this report were used by the Thaumasite Expert Group to establish interim recommendations for concrete mixes designed to minimise deterioration due to the thaumasite form of sulfate attack. 41 pages.