The preparation of magnetite particles is reported from the precipitation of Fe2+/Fe3+ with NH4OH in several kinds of microemulsions : AOT/n-heptane/water microemulsions and a new non-toxic microemulsion system whose phase behaviour and structure has been recently studied. It is shown that both the droplet size and the presence of interactions in the dispersed phase greatly influence the final size of the obtained particles. In addition, the size of the particles can. be increased by successive repetition of the reaction inside the droplets of the microemulsion.