We report an unusual enrichment and assembly of TiO2 nanocrystals at water/hydrophobic interfaces through oxidative hydrolysis of TiCl3 in water. The assembly is a spontaneous process that involves on-water inorganic reaction and assembly in the absence of any organic phases. In this process, TiO2 nanoparticles are preferentially produced at water/hydrophobic interfaces. When the surface tension of the aqueous phase is above a critical value, ca. 25-35 mN m(-1), these TiO2 nanocrystals can spontaneously accumulate at water/air interfaces to produce macroscopic sized sheets and tubes.