We have elucidated the reaction mechanism and the role of the reactive intermediates in the atomic layer deposition (ALD) of aluminum oxide from trimethyl aluminum in conjunction with O3 and an O2 plasma. In situ attenuated total reflection Fourier transform infrared spectroscopy data show that both -OH groups and carbonates are formed on the surface during the oxidation cycle. These carbonates, once formed on the surface, are stable to prolongedO3 exposure in the same cycle. However, in the case of plasma-assisted ALD, the carbonates decompose upon prolonged O2 plasma exposure via a series reaction kinetics of the type, A (CH3)fB (carbonates) fC (Al2O3). The ratio of -OH groups to carbonates on the surface strongly depends on the oxidizing agent, and also the duration of the oxidation cycle in plasma-assisted ALD. However, in both O 3 and O2 plasma cycles, carbonates are a small fraction of the total number of reactive sites compared to the hydroxyl groups.