Accurate monitoring of flow instabilities, which can occur when nonaqueous phase liquids (NAPLs) flow through porous media, is an important component of predicting the transport and fate of these compounds in the subsurface. In particular, flow situations in which three mobile phases (such as water, NAPL, and air) exist in the porous media are inherently complex. Unfortunately, the relatively low source intensities and the nontunable source energies make traditional dual gamma techniques unsuitable to study flow instabilities which can change within seconds. We present an alternate technique, which uses synchrotron X rays from the Cornell High Energy Synchrotron Source (CHESS) to measure three-phase fluid saturations on the time scale of seconds. Using the harmonic content resulting from X ray diffraction, we obtained a high-intensity X ray beam consisting of distinct tunable energies. Three-phase saturations were measured on 5-s timescales during fingering of light NAPL into regions of dry and water wet sandy soil. In the water wet soil the oil finger was less saturated, slower, and wider than the same finger in the dry soil. The results yield insights into the nature of three-phase preferential flow.¿ 1997 American Geophysical Union