Cataclastic deformation bands are significant discontinuities in sandstone reservoirs since they have dramatically reduced porosity and permeability relative to their host rock, despite their mm-scale displacements. Consequently, these discontinuities often have a large impact upon the flow of fluids at both micro and macro-scales. The effect of this impact in highly porous sandstone has been analyzed using a range of novel and conventional techniques, including pressure decay profile permeametry (PDPK) and positron emission tomography (PET). There are greater reductions in PDPK permeability in deformation bands relative to their host rock compared to conventional nitrogen permeametry measurements. This apparent discrepancy is the outcome of the higher spatial resolution of PDPK technique in measurements of much smaller rock volumes. There are greater porosity reductions using image analysis than conventional core techniques. These changes are reflected in a significant increase in irreducible water saturations (Swi) in deformation bands indicating much reduced fluid storage capacities. PET was used to monitor fluid flow as a function of pore volume of a sandstone plug containing deformation bands, demonstrating the direct effect of deformation bands as a potential barrier to fluid flow. The results of this study provide a detailed characterization of deformation bands at high resolution, which can be included in advanced reservoir simulation models. ¿ 2001 American Geophysical Union