Aluminium oxide (Al2O3) is a well-known thermally stable and insulating material employed in various applications as structural components and in thin film form. Al2O3 can be stabilized in several polymorphs in addition to an amorphous modification. The amorphous state of Al2O3 mainly exhibits interesting features, considering the absence of crystalline defects for the diffusion of charge carriers, which is paired with the difficulties in stabilizing crystalline Al2O3 during Physical Vapor Deposition (PVD). Therefore, in this study the influence of Si on the structural and chemical evolution of amorphous alumina-based thin films was systematically investigated using complementary advanced analytical techniques. The thin films (~ 350 nm) were produced via reactive High Power Impulse Magnetron Sputtering (HiPIMS) using Al-Si alloy targets in Ar/O₂ atmosphere, enabling the formation of amorphous alumina coatings with varying Si contents (see Fig. 1). To assess the phase evolution of these coatings, in-situ X-ray diffraction (XRD) was employed during vacuum annealing up to 1200 °C. To gain further insight into the role of Si in the phase stability and phase transformations of alumina at high temperatures, additional analytical techniques - including X-ray Photoelectron Spectroscopy (XPS), Scanning Auger-Meitner Electron Spectroscopy (AMES), and Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) - were employed to study the films prior to and after annealing at 1200 °C. In this contribution the results are summarized, and a detailed analysis of the structural and chemical characteristics of the thin films is presented.