Magnetic semiconducting thin films with high coercive field and remanence at room temperature are of primary importance for spintronic applications. We will use x-ray resonant diffraction to study the substitutional Fe doping of monoclinic CuO islands in epitaxy on SrTiO3(001). RXD, measured at the Fe K edge, allows to quantify the occupancy rate of the two cations on a same crystallographic site. At the same time GIXRD will allow to exclude the presence of spurious phases.