Clayey soils might lead to swelling, settlement and strength issues when considered as foundation or fill material. The use of additives to stabilize soils has been a major concern in the improvement of engineering characteristics. Upper Pliocene-aged clays cause various geotechnical problems in the residential area of Ankara. The aim of this study is to investigate the effect of lime, gypsum and lime with gypsum mixtures on the swell percent, swell pressure and unconfined compression strength of highly plastic clays in the Batikent region to specify an appropriate mixture ratio for soil stabilization. The optimum water content and maximum dry unit weight of clay were determined using standard compaction tests. Lime, gypsum and lime with gypsum mixtures were re-compacted in order to evaluate their effect on the swell percent, swell pressure and unconfined compression strength over curing time. Swell percent and swell pressure reduce with increasing additive percent over time, while the strength increases. The lowest improvement ratio is determined for gypsum. The optimum mixture is concluded to be 6 % lime, which corresponds to a 99.55 % decrease for swell percent, 98.98 % decrease for swell pressure and 191.87 % increase for the unconfined compressive strength for 90 days.