Mesoporous silica hollow microspheres were quickly synthesized with a simple reaction medium which only contains tetraethylorthosilicate (TEOS) as silica source, octylamine (OA) as template and water. It is proposed that a dispersion of TEOS-in-water is formed when water is added into the mixture of TOS and OA. Water becomes basic due to dissolution of OA. Fast hydrolysis and condensation of TEOS then occurs at the interface of droplets lead to the formation of silica shells. Hollows are formed due to the volume contraction caused by the large difference in the densities of condensed silica and liquid TEOS. In addition, OA assembles with silica, templating the nanopores in the silica shells.