This paper reports a study on the effect of a high-reactivity metakaolin (HRM) on the sulfate resistance of mortars. Mortar bars containing 0-25% HRM were prepared in accordance with ASTM C 1012-89 with three cements of varying tricalcium aluminate content. After 6 months exposure to a 5% N2SO4 solution, mortar bars incorporating any level of HRM (5-25%) as a partial replacement for a high tricalcium aluminate cement may be considered "moderate sulfate resistant". Mortar bars made with HRM and a moderate or low tricalcium aluminate cement may be considered "high-sulfate resistant". Beyond 6 months, however, it was found that insufficient levels of HRM may, in fact, reduce the sulfate resistance of some cements, and that 15% HRM or more, may be required for long term sulfate resistance, depending on the tricalcium content of the cement.