The effect of acid rain on mortar durability is not well understood. In particular, little work has been done on the reaction of magnesium hydroxide in dolomitic Type S hydrated lime with sulfuric acid compounds. In this study, this reaction is investigated using two different exposure situations, each involving distilled water and sulfuric acid (4.5 pH) as leachants. In the first situation, the leachability of the Type S hydrated lime is examined through packed-bed column testing. In the second situation, hardened mortar samples were placed in both distilled water and sulfuric acid (4.5 pH) solution for one week. Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP) was used to determine the magnesium content of the raw materials, hardened mortars and leachate. Magnesium levels of the leachate samples were consistently low and did not appear to be affected significantly by the presence of sulfuric acid.