The aluminum precursor plays a crucial role in the Al2O3 ALD process. To date, trimethylaluminum (TMA) is one of the most widely used precursors in experimental and theoretical studies. However, its application at industrial scale can pose safety risks since it is pyrophoric and extremely reactive with water. Aluminum alkoxides offer a promising alternative, but have received far less attention. A combined theoretical and experimental investigation is carried out on the Al2O3 ALD process using aluminum triisopropoxide (ATIP) as a prototypical example of Al-alkoxide precursors. The experimental results pointed out that the thermal ALD process using ATIP and water has a maximal growth per cycle (GPC) of 1.8 Å/cycle at temperatures of 150 to 175 °C. On the basis of the in situ mass spectrometry analysis and DFT calculations, the formation of the alumina film mainly occurs during the water pulse by ligand exchange reactions between water and adsorbed precursors, while during the ATIP pulse only adsorption of ATIP and/or its dissociation occur. Design of heteroleptic precursors containing alkoxide group as basic ligand is challenging, but greatly promising for future industrial scale Al2O3 ALD.