Permeability reductions, effected in a porous medium consisting of 40-60 μm glass spheres by passage of emulsions, are reported for Na oleate-stabilized emulsions at different electrolyte concentrations, and for an emulsion containeing, Aerosol OT (Na dioctyl sulphosuccinate; AOT) at low electrolyte concentration. All emulsions show a pronounced permeability reduction, but for Na oleate-stabilized emulsions, in the absence of added electrolyte, substantial amounts of disperse phase have to be passed through in order to reduce the permeability to about 35% of its initial value. Addition of electrolyte above the critical coagulation concentration (CCC) leads to more rapid permeability reduction. AOT-stabilized emulsions, even in the absence of added electrolyte, reduce the permeability of the porous medium in a way comparable to that found for Na oleate-stabilized emulsions with [NaCl] > CCC. This is ascribed to coagulation of the AOT-containing emulsion even in the absence of added electrolyte. This coagulation is indeed observed on passing the AOT-stabilized emulsions through the porous medium. The reduced stability of the AOT-stabilized emulsion can be accounted for by the low interfacial tension in lhis emulsion, permitting emulsion destabilization through droplet deformation.