The ever-increasing number of failures of industrial structures due to the heaving of acid-contaminated foundation soils has necessitated a better understanding of soil behavior under changing and extreme environmental conditions. Thus, this paper attempts to micro-mechanistically explain the swelling characteristics of soils contaminated with inorganic acids. Three soils with widely varying physical and chemical properties, namely, natural black cotton soil and commercially available bentonite and kaolin clay, were selected for the investigation. Special Teflon-made oedometer cells, which are entirely non-reactive to acid, were used to assess the swelling behavior. The soils were inundated with two concentrations of sulfuric acid and phosphoric acid and allowed to swell. The results indicate that, in montmorillonitic soils, the type of cation in the exchangeable complex plays a dominant role in governing the swelling behavior of clays during acid contamination. The mineralogical changes due to cation exchange reactions, along with the partial mineral dissolution, resulted in the acid-induced swelling in montmorillonitic soils. In the kaolin clay, the face-to-edge association of the particles due to the adsorption of H⁺ by broken edges led to an increase in swelling along with mineralogical changes.