Atomic layer deposition (ALD) of alumina (Al2O3) using water and aluminum trichloride (AlCl3) is studied using density functional theory (DFT). The atomistic mechanisms of the two deposition half-cycles on Al2O3OH* and Al2O3-Cl* surface sites include the formation of stable intermediates and result in high barriers for HCI formation. Increasing the temperature reduces the stability of the intermediates but also increases the desorption rate of adsorbed precursors. Both half-reactions are qualitatively similar to the corresponding reactions of ALD of HfO2 and ZrO2 from H2O and HfCl4 and ZrCl4, respectively, but differ significantly from the reactions of ALD of Al2O3 from AI(CH3)(3) and H2O. Although the high electronegativity of Cl increases the stability of the dative-bonded trapped intermediates in the Zr and Hf chloride cases, the inductive effect is stronger for AlCl3, increasing the stability of adsorbed AlCl3 by 29 kcal/mol relative to the adsorption energy of Al(CH3)(3). Furthermore, the ligand-exchange barrier is 26 kcal/mol higher for AlCl3 than for Al(CH3)3, thus reducing the reaction kinetics and increasing the ALD temperature for the AlCl3/H2O system. We have also found a new reaction pathway which involves ligand-exchange without metal deposition which we call nongrowth ligand-exchange. This pathway is more competitive in ALD using AlCl3 as the metal precursor than when using AI(CH3)3 and reduces the ALD growth rate. These results provide additional physical insight into the nature of these reaction mechanisms and demonstrate that the Cl ligands have a larger impact on the reaction energetics than the metal atom for the ALD of these high-K oxides.