Gels have often been injected into production wells in an effort to reduce water production without seriously damaging hydrocarbon productivity. To be effective, the gels must significantly reduce permeability to water while causing minimal reduction in permeability to oil. Many gels exhibit this disproportionate permeability reduction, but the property has not been as predictable as we would like. If the mechanism for this property were understood, gel treatments could be applied with greater reliability. To determine this mechanism for a Cr(III)-acetate-HPAM gel, we used X-ray computed microtomography (XMT). As an important prelude to investigating gel behavior in porous media, XMT was used to characterize imbibition and drainage processes in water-wet and oil-wet porous media before gel placement.