Simultaneous treatment of two industrial wastes, i.e. the electroplating sludge and the spent pickling liquor, to produce valuable spinel ferrite with high crystallinity and high saturation magnetization through the hydrothermal process has been investigated. After the hydrothermal treatment, the electroplating sludge can only produce poorly crystallized ferrite with high content of impurities. However, co-hydrothermal treatment of the electroplating sludge and the spent pickle liquor in a suitable mixing ratio results in a well crystallized ferrite product, with all heavy metals being fixed into the ferrite lattice. In order for the ferrite product to have a maximum saturation magnetization (41.42 emu g(-1) in this research), the response surface methodology (RSM) has been employed to derive the optimal experiment condition, e.g. hydrothermal treatment at 300 degrees C for 6 h with the initial pH adjusted to 10. This optimal experimental condition has been further proven valid when applied to a different kind of electroplating sludge. In addition, the magnetization of ferrite products can be further enhanced to 58.34 emu g(-1) by acid washing to remove the impurity (calcite). This research thus shows an effective way to transfer two common industrial wastes into valuable ferrite materials. (C) 2010 Elsevier B.V. All rights reserved.