This paper describes the use of an X ray computerized tomography scanner for noninvasive, three-dimensional, high-resolution imaging of air distribution patterns during air sparging in water-saturated sandpacks. Experiments were performed in Plexiglas cylindrical cells using well-defined Ottawa sands. The reconstructed images provide detailed porosity and air saturation distributions previously unavailable. Two classes of behavior were observed. In high-permeability (106 darcy) uniform sandpacks a relatively few, discrete, and tortuous air pathways were formed. Steady state was rapidly attained, and the intrachannel air saturations were low (0.10). In contrast, low-permeability (3 darcy) sandpacks increased the breadth of air contact, delayed attainment of steady state, and displayed high air saturations (0.50). The concept of a hydrodynamic stagnation saturation and standard one-dimensional fractional flow theory proved useful in explaining the experiments. Numerical simulation confirms the explanations, even when capillary pressure and compressibility effects are included. ¿ American Geophysical Union 1996