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 O-3 and an O-2 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 prolonged O-3 exposure in the same cycle. However, in the case of plasma-assisted ALD, the carbonates decompose upon prolonged O-2 plasma exposure via a series reaction kinetics of the type, A (CH3) -> B (carbonates) -> C (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 O-2 plasma cycles, carbonates are a small fraction of the total number of reactive sites compared to the hydroxyl groups.