In situ time-resolved small-angle X-ray and neutron scattering have been used to investigate the mechanism of a bulk precipitation of a hexagonally ordered ZrO2/surfactant mesophase. Using hexadecyltrimethylammonium bromide (CTAB) as a structure-directing agent the precipitation results from the addition of sulfate to an oxychloride zirconium/CTAB water solution. The precipitate obtained after a few seconds is organized in a 2D hexagonal network. There is no preexisting cylindrical micelle in the precursor solution, and the micelles act as a reservoir for monomer to feed the building grain. The precipitation occurs through a two-step mechanism: a random nucleation/growing process and a first-order reorganization from locally ordered cylinders to a hexagonal lattice in the grain at its final size.