[en] The thermal instability and silicidation behaviors of ultrathin HfO_x on Si(001) were studied by using scanning tunneling microscopy (STM) with in-situ X-ray photoelectron spectroscopy (XPS). A 1.5-nm-thick HfO_x layer was grown on Si(001) by using Hf deposition in an oxidizing ambient. In the low temperature (600 ∼ 800 .deg. C) region, hafnium oxide near the interface region easily reacted with the substrate silicon ions in order to form a silicate structure (Hf-O-Si bonding units). When the SiO evaporation proceeded in the high-temperature (≥ 900 .deg. C) region, silicon migration from the substrate became kinetically remarkable and transformed the Hf-O-Si units into Hf-Si bonds with some remnant Si-O bonds. In contrast with the multi-phase feature of crystalline HfSi₂ that has been reported to occur by annealing hafnium metal on ultrathin SiO₂, our STM study reveals the formation of 'irregularly shaped' silicides upon silicidation. These silicides contain some oxygen in the form of Si-O bonds, so they exhibit a slightly insulating behavior in the spectroscope mode of STM.