Hierarchical Fe₂O₃@WO₃ nanocomposites with ultrahigh specific areas, consisting of Fe₂O₃ nanoparticles (NPs) and single-crystal WO₃ nanoplates, were synthesized via a microwave-heating (MH) in situ growth process. WO₃ nanoplates were derived by an intercalation and topochemical-conversion route, and the Fe₂O₃ NPs were in situ grown on the WO₃ surfaces via a heterogamous nucleation. The water-bath-heating (WH) process was also developed to synthesize a Fe₂O₃@WO₃ nanocomposite for comparison purposes. The techniques of X-ray diffraction (XRD), X-ray photoelectron spectrum (XPS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to characterize the samples obtained. The results show that α-Fe₂O₃ NPs with a size range of 5–10 nm are uniformly, tightly anchored on the surfaces of WO₃ nanoplates in the Fe₂O₃@WO₃ samples obtained via the MH process, whereas the α-Fe₂O₃ NPs are not uniform in particle-sizes and spatial distribution in the Fe₂O₃@WO₃ samples obtained via the WH process. The BET surface area of the 5wt%Fe₂O₃@WO₃ sample derived by the MH process is as high as 1207 m² g⁻¹, 5.9 times higher than that (203 m² g⁻¹) of the corresponding WO₃ nanoplates. The dramatic enhancement in the specific surface area of the Fe₂O₃@WO₃ samples should be attributed to the hierarchical microstructure, which makes the internal surfaces or interfaces in aggregated polycrystals be fully outside surfaces via a house-of-cards configuration, where the single-layered and disconnected Fe₂O₃ NPs are tightly anchored on the surfaces of the WO₃ nanoplates. The gas-sensing properties of the Fe₂O₃@WO₃ sensors were investigated. The gas-sensors based on the Fe₂O₃@WO₃ obtained via the MH process show a high response and selectivity to H₂S at low operating temperatures. The 5%Fe₂O₃@WO₃ sample shows the highest H₂S-sensing response at 150 °C. Its response to 10 ppm H₂S is as high as 192, 4 times higher than that of the WO₃-nanoplate sensor. The improvement in the gas-sensing performance of the Fe₂O₃@WO₃ nanocomposites can be attributed to the synergistic effect in compositions and the hierarchical microstructures with ultrahigh specific surface areas.