THE mesoporous molecular sieves developed recently(1,2) have stimulated a great deal of interest in large-pore materials for selective rs catalysis and other applications(6). The synthesis of these materials involves the use of ionic surfactants which interact with the inorganic ions in a cooperative process to form a range of ordered mesostructures(7,8). Many of these same surfactants can be intercalated into layered inorganic materials(9,10), and we explore here the possibility of using intercalated surfactants for the templated synthesis of structures within the interlayer spaces (galleries) of clays. The surfactants act in concert with aqueous silicate species to form a silica framework between the layers. Removal of the surfactant by calcination then leaves a mesoporous solid with thermally stable pores of widths in the range 14-22 Angstrom. These materials, which we call porous clay heterostructures, might provide new opportunities for the rational design of heterogeneous catalysts.