Abstract The alkali activation of metakaolin is a way of producing high strength cementitious materials. The processing of these materials has been the subject of numerous investigations. The present paper describes the results of a research project initiated to study the stability of these materials when exposed to aggressive solutions. Prisms of mortar made of sand and alkali-activated metakaolin were immersed in deionized water, ASTM sea water, sodium sulfate solution (4.4% wt), and sulfuric acid solution (0.001 M). The prisms were removed from the solutions at 7, 28, 56, 90, 180, and 270 days. Their microstructure was characterized and their physical, mechanical, and microstructural properties were measured. It was observed that the nature of the aggressive solution had little negative effect on the evolution of microstructure and the strength of these materials. It was also found that the 90-day and older samples experienced a slight increase in their flexural strengths with time. This tendency was most pronounced in those samples cured in sodium sulfate solutions. This behavior may be related to the change in microstructure of the cementitious matrix of the mortars cured longer than 90 days. Some of the amorphous material present had crystallized to a zeolite-like material belonging to the faujasite family of zeolites.