Mesoporous NiCo2O4 spinel has been synthesized by nanocasting, using SBA-15 and KIT-6 silica as hard templates. Two temperatures of calcination were applied for the conversion of the spinel precursors (metal nitrates) into NiCo2O4. At 375 °C the pure spinel was obtained, whereas at 550 °C NiO impurities were detected. The mesoporous powders obtained were characterized by transmission electron microscopy, X-ray diffraction, Brunauer−Emmett−Teller analysis, and magnetic measurements. In addition, their thermal stability was assessed by post heat-treating the materials at 550 °C: the mesostructure of the pure spinel (calcined at 375 °C) collapsed, leading to the disruption of the porous network, while the mesoporosity of the powders calcined at 550 °C was preserved. In all cases, the post-thermal treatment induced the segregation of NiO; the final NiO amount was found to be in the 7−10 wt % range.