Abstract Reactions between HfO2 and Si in HfSiO films during deposition and post-annealing have been studied. Intermixing of HfO2 and Si is achieved by radio frequency sputtering with HfO2/Si compound targets, and post-annealing is used to promote the reaction at different temperatures. The structural characteristics of the mixture, HfSiO films, are analyzed by X-ray photoelectron spectroscopy and X-ray diffraction, and a careful assessment of chemical states is performed for precise identification. XPS results show that with ratios of Si:Hf ranging from 0 to 0.3 in HfSiO films, Si fully reacts with HfO2 to form silicate during deposition. However, SiO2 appears when the ratio of Si:Hf rises to 1.2. When the annealing temperature reaches 600°C, decomposition of hafnium silicate is observed and hafnium silicide forms in the bulk of the films. XRD results reveal that HfSiO films remain amorphous with the annealing temperature below 600°C but crystallize at 800°C.