Micrometer-sized hierarchical Sn3O2(OH)(2) octahedra, which are self-assembled one inside the other, resembling "Russian doll" organization, have been obtained by a metalorganic approach. This synthesis is based on the controlled hydrolysis of [Sn(NMe2)(2)](2) in the presence of an alkylamine ligand in an organic solvent (THF). The water content of the medium proved to be a key parameter for the formation of these multi-walled octahedra. The resultant structures have been used as gas-sensitive layers on micromachined silicon devices. During in situ heating, Sn3O2(OH)(2) is oxidized to SnO2 while retaining the initial morphology. The sensors present outstanding dynamic responses at very low CO concentrations (7% and 67% resistance variation to 0.25 and 20 ppm CO, respectively, at an operating temperature of 500 degrees C). This superior gas-sensing performance is closely related to the unique microstructure of the SnO2 multi-walled octahedra.