In this paper, thermocouple psychrometric technique is used to measure soil suction and reliability of swell percentage prediction method based on soil suction data is evaluated. Kaolinite-bentonite clay mixtures were prepared to obtain expansive soils in a wide range of plasticity indices. Suction measurements using thermocouple psychrometer technique and free swell tests in oedometers were made on statically compacted specimens. Soil suction-water content relationships were evaluated. The results indicated a linear relationship between log suction and swell percentage measured in oedometer. It has been shown that the predicted swell percentages by using suction test results underestimate the ultimate swell percentages in oedometer free swell tests.