The acid resistance of 60-day cured alkali-activated slag (AAS) mortars, in comparison with Portland cement, is assessed. Specimens are exposed to hydrochloric, nitric and sulphuric acids at pH 3.0, and acetic acid (CH3COOH) at pH 4.5, for 150 days, with specimens also immersed in water as a control. Negligible changes in compressive strength are identified in Portland cement and AAS binders during exposure to mineral acids, while compressive strength increases during immersion in water. However, exposure to CH3COOH reduces strength, and increases pore volume, in both Portland cement and AAS mortars. AAS performs better than Portland cement in CH3COOH, associated with the lower initial permeability of the specimens, along with the low CaO/SiO2 ratio typical of AAS. Decalcification of the AAS binder through formation of calcium acetate leaves an aluminosilicate type gel that can hinder the further ingress of acids, contributing to the high acid resistance of this binder.