X-ray computed tomography (CT) offers distinct advantages to study fundamental physical processes of water movement and contaminant transport in porous media. Tomography provides nondestructive and noninvasive cross-sectional or three-dimensional representations of porous media and has the potential to measure phase distribution and species concentration at the pore scale. Sources of error are discussed for the application of industrial microfocus CT to quantitative studies of flow and transport. Specifically, effective resolution and measurement uncertainties due to photon randomness are considered for a miscible displacement experiment. A calibration method for the measurement of solute concentration is proposed that accounts for the effect of beam hardening, which is characteristic for polychromatic industrial X-ray sources. The results of an X-ray microfocus CT experiment are presented, emphasizing the need to correct for beam hardening and describing the inherent spatial variability of solute breakthrough through a glass-bead porous medium with an effective spatial resolution of approximately 85 &mgr;m. ¿ 2000 American Geophysical Union