The structural voids in vertisols contain easily available water for plants and their volume can be calculated from the shrinkage curve. Access by plants to that water depends also on the geometric arrangement of the pores so that the water can flow through them. We have devised a method for studying the structural porosity by casting the pores in resin. The intraprism pore space of wet soil clods is impregnated with a UV fluorescent polyester resin under vacuum. When this has set we use the swelling properties of the clay to separate the clay matrix from the resin. A cast so obtained is the real three-dimensional solid reproduction of the structural porosity. This representation of the pore system is' easier to study than results from computerized reconstitution of the three-dimensional space from two dimensional images of soil in thin sections. Channels, packing pores and planar voids can be observed directly in three dimensions as the method saves the integrity and continuity of pores as small as 10 microm in diameter. The geometry of the cast shapes agrees with the interpretation of shrinkage and moisture characteristic curves. The method offers direct qualitative observation of pore organization and volume measurements of the intraprism structural porosity in vertisols.