Nanostructures of SnO2 including nanoparticles, 1 nanowires, 2 nanobelts, 3 and nanotubes 4 have been widely used in many fields, such as gas sensors, solar cells and lithium batteries. Recently, hierarchical and/or hollow SnO2 micro-and nanostructures have attracted much interest because of their widespread potential applications such as gas sensors. 5 We present here the formation of self-assembled tin oxohydroxide (Sn3O2(OH)2) supercrystals organized in a "Russian-doll" structures and obtained by an organometallic synthesis, with finely tuned water addition. These supercrystals have been characterized by transmission and high resolution transmission electron microscopy, field-emission scanning electron microscopy, X-ray powder diffraction, and Fourier transform infra-red spectroscopy. These super-octahedra have been used as gas sensitive layers deposited on silicon devices. After in-situ heating, Sn3O2(OH)2 easily oxidizes into SnO2 while retaining the initial morphology and porosity (fig.1). The response of the sensors to reducing and oxidizing gases has been measured at relative humidity (RH) of 50%. At 500°C and under very low CO concentrations (0.25 to 20 ppm), the sensors present an outstanding dynamic response (7% and 67% of resistance variation) (Fig. 2). These unprecedented detection performances are strongly relied to the hierarchical microstructure of SnO2 supercrystals. These sensitive layers open the way to the development of metal oxide devices dedicated to extremely low gas concentration determination.