Understanding soil swelling is hampered by the difficulty of simultaneously measuring water content and bulk density. A number of studies have used dual-energy gamma rays to investigate soil swelling. The long counting time of this technique makes it impracticable for studying the rapid changes in moisture content and soil swelling shortly after infiltration is initiated. In this paper, we use the dual-energy synchrotron X ray to measure, for the first time, the water content and bulk density changes during the fast, initial phase of the swelling process. Ponded infiltration experiments were performed with two soils: a bentonite-sand mixture and a vertisol. Swelling curves and hydraulic diffusivity were determined. Deformation was very rapid immediately after water application and then became progressively slower. The hydraulic diffusivity decreased with time, which can partially explain the very rapid decrease in infiltration rates observed in the field. ¿ 1998 American Geophysical Union