The aim of this paper is to propose a model obtained through experimental design which represents the synthesis of magnetite by oxidation-precipitation of sulfuric liquors from steel pickling. The studied variables are pH (5.5-6.5), stirring speed (250-1,000 rev.min(-1)) and flow of air used as oxidizer (5-101.min(-1)). The model is valid within the experimental ranges and for a temperature of 70degreesC. The proposed model is validated through statistical methods. The influence of each variable is studied related to the reaction mechanism and the optimal value of each one is determined.