Using a capillary video-micro scopy technique, the formation of ZrO2 precursors with the two reverse-emulsion precipitation method was visually studied by preparing the reverse W/O emulsions within a thin-walled, transparent, cylindrical microtube. Two types of aqueous droplets, i.e., one containing ZrO(NO3)(2) Solution and the other containing ammonia solution, were prepared in the oil phase to produce ZrO2 precursors. It was found that the ZrO2 precursors could hardly be formed by the coalescence of the emulsion droplets, even if they were in contact with each other. Instead, the formation of ZrO2 precursors was mainly induced by ammonia diffusion across the oil phase. Interestingly, the growth of ZrO2 precursors did not occur within any of the prepared aqueous droplets, which are generally considered to be space-limiting microreactors, as expected from much previous research. The ZrO2 precursors were mostly formed by the distortion and split of the aqueous droplets containing ZrO(NO3)(2) Solution. Increased surfactant content in the oil phase slowed the formation of the ZrO2 precursors, probably due to the increased viscosity, which can hinder the distortion and split of the aqueous droplets. In addition, the growth of ZrO2 precursors was also retarded by the increase of the size of the aqueous droplets.