Determination of swelling pressure exerted by expansive soils is important for designing structures such as shallow foundations, shallow tunnels, retaining walls, canal linings and underground conduits in such soils. The axial deformation and swelling pressure under zero lateral deformation can be determined by conventional oedometer using two identical samples. However, this test doesn't permit a synchronized measurement of both the axial and lateral swelling pressures on the same sample. The Ankara clay, which is an overconsolidated clay, rich in smectite group clay minerals, and partly covers the southern part of Ankara. the capital of Turkey, causes damage to light structures located at the surface and at shallow depths. A comprehensive study was conducted (i) to determine the swelling parameters (swelling pressure and swelling percentage) of Ankara clay both in vertical and lateral directions using a thin wall oedometer ring (providing synchronized measurement of swelling parameters in both directions), (ii) to compare lateral and vertical swelling parameters of the clay for the assessment of swelling anisotropy, and (iii) to investigate the effect of microstructure of Ankara clay on its swelling behavior using Scanning Electron Microscope (SEM). The experimental results revealed that the swelling parameters measured in vertical direction were greater than those measured in lateral direction. The ratio between the swelling pressures in lateral and vertical directions was found between 0.34 and 0.98. The results of the SEM analyses suggested that the clay minerals in Ankara clay show horizontal and/or nearly horizontal sheeting and have stepped face-to-face structure. This arrangement indicates that the swelling parameters in vertical direction should be greater than those in lateral direction. It also confirms the experimental results and shows the influence of the microstructure of the clay on swelling anisotropy. (C) 2009 Elsevier B.V. All rights reserved.