Expansive soils have the tendency to undergo volume change behavior and cause large uplift pressures and upheaval of structures built on them. Avoiding expansive soil in most of the cases is not practically possible, hence stabilization practices are undertaken to treat them. Chemical stabilization through lime is the most sought after treating method. In this study, soil from an expansive soil site, Siruseri near Chennai, was treated with lime to understand the microstructural changes taking place and its influence on the macro structural properties. X-ray diffraction, SEM and microporosimetry were the techniques used to understand the microstructural properties of lime treated soils. The expansive soil was treated with different percentage of lime, 2, 4, 6, and 8 % and cured for 3, 7, 14, and 28 days. The concept of lime modification optimum (LMO) was used as a reference to understand the progression of lime reactions. The change in the physical properties of the soil was determined through UCC, chemical batch testing, pH and chemical conductivity. The lime addition higher than the LMO brings about more permanent pozzolanic reactions which cause tremendous increase in strength. The time dependency of the reactions was observed through the change in pH values, conductivity and batch test results. At the microstructural level, lime addition reduces the specific surface area and increases the pore size. The SEM results also revealed larger clusters and aggregation of the clay particles. © 2013 Indian Geotechnical Society.