In this paper, we report infrared absorption studies of HfO2, HfO2/Si interface and Hf(1−x)SixOy. Both HfO2 crystallization and SiO2 formation at the interface can be clearly detected in the absorption spectra in the far and middle infrared regions, respectively. By measuring the intensity change and the peak shift of infrared absorption spectra as functions of annealing temperature and time together with XRD patterns, we discuss a difference of the amorphous structure between HfO2 and SiO2, and also show an evolution of HfO2 crystallization in the monoclinic phase up to 1000 °C. On the other hand, it is shown that the interfacial SiO2 layer is qualitatively similar to the thermally grown SiO2. Furthermore, it is demonstrated that a Si incorporation into HfO2 film significantly changes the IR absorption spectra, and that the Hf(1−x)SixOy film is phase-separated with an appearance of modified monoclinic phase by higher temperature annealing.