Steel pickling liquors are one of the main environmental problems of steel making. Currently, there are several processes for the treatment of sulphuric liquors, although most recover only acid and haematite (alpha-Fe2O3). We propose an oxyprecipitation process for this recovery, allowing several kinds of iron oxide or oxyhydroxide to be obtained. The aim of this paper is to determine the kinetic control and reaction mechanism. The influence of different variables is evaluated and two experimental ranges for the synthesis of goethite (alpha-FeOOH) and magnetite (Fe3O4) are defined. A systematic study is carried out in these experimental intervals, and the results are analysed in order to determine the reaction order and the type of kinetic control. Finally, from these data, morphological and crystallinity studies and new experiments, the reaction mechanism is proposed.