A method to precipitate nanoparticles using a miniemulsion technique is described, in which miniemulsion droplets between 100 and 1000 nm in size serve as nanoreactors enabling both the control of particle formation and particle growth. The application of miniemulsion droplets to synthesise nanoparticles comprises three advantages: first, the size of the precipitated particles is limited by the reactant concentration within the emulsion droplet; second, particle agglomeration is prevented as nanoparticle collision outside the nanoreactor is avoided; and third, easy technical scale up can be realized by increasing emulsion volume and thus the number of nanoreactors, while local conditions within the reactors are not changed. The miniemulsion technique is an easy scalable process which allows defined synthesis of particles by precipitation reactions. The miniemulsion technique involves first the preparation of a stable water-in- oil miniemulsion by high pressure homogenisation. Whereas a water soluble reactant is provided within the aqueous droplets, another oil- as well as water-soluble reactant can be introduced to the emulsion after homogenisation. The precipitation reaction is induced by the diffusion of the second reactant into the emulsion droplet. Together with this contribution, a method is described and discussed which uses a high pressure homogenisation process to produce stable water-in-oil miniemulsions serving as a reaction medium to precipitate metal oxides. A method to precipitate nanoparticles using a miniemulsion technique is described, in which miniemulsion droplets between 100 and 1000 nm in size serve as nanoreactors enabling both the control of particle formation and particle growth. The application of miniemulsion droplets to synthesise nanoparticles comprises three advantages: first , the size of the precipitated particles is limited by the reactant concentration within the emulsion droplet; second, particle agglomeration is prevented as nanoparticle collision outside the nanoreactor is avoided; and third, easy technical scale up can be realized by increasing emulsion volume and thus the number of nanoreactors, while local conditions within the reactors are not changed. The miniemulsion technique is an easy scalable process which allows defined synthesis of particles by precipitation reactions. The miniemulsion technique involves first the preparation of a stable water-in- oil miniemulsion by high pressure homogenisation. Whereas a water soluble reactant is provided within the aqueous droplets, another oil-as well as water-soluble reactant can be introduced to the emulsion after homogenisation. The precipitation reaction is induced by the diffusion of the second reactant into the emulsion droplet. , a method is described and discussed which uses a high pressure homogenisation process to produce stable water-in-oil miniemulsions serving as a reaction medium to precipitate metal oxides.