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Abstract Experiments by three methods to determine the swelling pressure of clays showed that the conventional consolidation test gives an upper bound value, the method of equilibrium void ratios for various consolidation pressures gives the least value, and tests by the constant volume method give intermediate values. However there is no definite relationship between the three methods. The paper also shows that time effects involved in conducting the tests and the effect of stress path are both significant. Time versus swelling, and to a greater extent, time versus pressure relations can be represented by a rectangular hyperbola, which can be used to predict the ultimate swelling and the swelling pressure, respectively. Tests with varying initial conditions of molding moisture content and density have exhibited a unique relationship between swelling pressure and the initial dry density for black cotton soil. This result is in line with the osmotic pressure theory. The molding water content has marginal effect on the swelling pressure.