This paper studies the swelling of highly consolidated mudstones by theoretical considerations and laboratory experiments. A key assumption was made that saturated and uncemented clays behave as heavily dense colloid without direct contacts among solid particles. It leads to an important conclusion that the swelling pressure acting on adsorbed interparticle water-films is equivalent to the effective stress. This so-called clay-colloid concept is validated by various swelling experiments on two kinds of mudstones, the Callovo-Oxfordian argillite in France and the Opalinus clay in Switzerland. In the tests, water adsorption-desorption, swelling pressure and strain were measured on the samples at various suctions and load-controlled conditions. Results suggest that: (1) the mudstones can take up great amounts of water from the humid environment, much more than the water content in the natural and saturated states; (2) the swelling pressure increases with water uptake to high levels of the overburden stresses at the sampling depths of 230 to 500 m, indicating that the adsorbed water-films are capable of carrying the lithostatic stress; and (3) the large amount of water uptake causes a significant expansion of mudstones even under the lithostatic stresses.