Low rise buildings are particularly vulnerable to ground movements sourced from swelling and shrinking of the expansive clay soils. Geotechnical engineers have long recognized that swelling of expansive clays caused by moisture variation may result in considerable damage to the overlying structures, and engineers should take them into the consideration. The literature contains a vast number of stabilizing techniques such as lime, cement and fly-ash for treatment of expansive clay soils. However the use of gypsum as a stabilizing agent is currently not clear. This paper deals with the performance of the gypsum as an additive for treatment of the expansive clay soils by means of swell potential and strength. Optimum water content for the best compaction of the bentonite was first determined by standard compaction tests. Different quantities of gypsum such as 2.5%, 5%, 7.5%, and 10% by mass were added to bentonite and compacted in optimum water content obtained. Atterberg limits, free swell and unconfined compressive strength tests were performed on treated and untreated samples, after a curing period of 7 days. Obtained changes in the plasticity, swell percent and strength parameters of treated and untreated samples indicated that gypsum can be used as a stabilizing agent for expansive clay soils, effectively. (C) 2009 Elsevier B.V. All rights reserved.