The only easy way to prepare amorphous alumina is via thin film deposition. For this reason, the disorder in amorphous alumina has not yet been fully investigated. We have used very high-field (20 T) solid state 27Al NMR spectroscopy to analyze the structural modifications of amorphous alumina thin films with deposition temperature (Td). The films were deposited by metalorganic chemical vapor deposition in the Td range of 360–720 °C. Depending on Td, film composition is either AlO1+x(OH)1–2x (0 ≤ x ≤ 0.5) or Al2O3. From 27Al 1D magic angle spinning (MAS) and 2D multiple-quantum magic angle spinning (MQMAS) NMR analyses, the films grown between 360 and 600 °C contain between 38 and 43 atom % of 5-fold coordinated aluminum sites ([5]Al). The percentages of [6]Al and [4]Al sites vary spectacularly, reaching their respective minimum (5 atom %) and maximum (54 atom %) around 515 °C. The analysis of a very thin film (85 nm) of Al2O3 reveals the presence of metallic aluminum at the interface with the substrate and suggests that the respective percentages of [n]Al sites slightly differ from those in thicker films. The observed Td dependence of amorphous alumina structure can be correlated with that of film properties previously reported, namely, Young’s modulus, hardness, and corrosion protection.