Atomic layer deposition (ALD) is considered one of the primary candidates for the deposition of ultrathin high-k metal oxides e.g. Al2O3, essential for the application as insulator in 3D capacitors and gate dielectrics in semiconductor industry. To allow for depositions at low temperatures and to improve the film quality of ALD metal oxides, oxygen sources other than H2O are researched. In precious work the plasma-assisted ALD process of Al2O3 employing Al(CH3)3 (TMA, trimethylaluminum) and an O2 plasma was investigated. In particular gas phase reaction products of the Al(CH3)3 exposure and O2 plasma properties were studied, thereby leaving the exact surface chemistry involved in the film growth unresolved. In order to further unravel the surface chemistry and further exploit the benefits of this ALD process, the determination of the chemical surface groups involved in plasma-assisted ALD of Al2O3, is essential. The ALD reactor was adapted to study both plasma-assisted and thermal ALD of Al2O3 for substrate temperatures between 25 ºC and 150 ºC, on the same setup. In situ transmission infrared spectroscopy was installed and used to probe ALD surface chemistry on KBr windows mounted in the reactor walls. Most experiments focused on the plasma-assisted ALD process, however, since thermal ALD is a well known process, a direct comparison of plasma-assisted and thermal ALD was performed at 150 ºC. During plasma-assisted ALD of Al2O3 -OH and -CH3 surface groups were predominantly formed after the O2 plasma and TMA exposure, respectively. Similar surface groups were found for thermal ALD of Al2O3 with H2O as an oxidant. At lower deposition temperatures (100 ºC) higher -OH densities and equal -CH3 densities are involved in the surface reactions, which indicated more bifunctional chemisorption of TMA on the hydroxylated surface at lower temperatures. After the O2 plasma exposure at 25 ºC, carbon related surface groups were observed and incorporated in the film. However, these impurities in the film could be reduced by extending the O2 plasma exposure time, suggesting that the combustion like removal of the -CH3 surface groups by the plasma was initially not complete. This process is therefore supposed to be partially temperature driven. Besides surface groups, the formation of CH4 as a gas phase reaction product during the TMA exposure was confirmed by means of in situ infrared transmission spectroscopy. The amount of CH4 produced has been estimated from the data and supports the possibility of bifunctional chemisorption of TMA. By combining the results on the surface groups and the gas phase reaction products, the reaction mechanism of plasma-assisted ALD of Al2O3 was further resolved and more insight was obtained into the plasma-assisted ALD process of Al2O3 at low substrate temperatures.