In atomic layer deposition (ALD) of Al2O3 thin films using trimethylaluminum (TMA) and water, it is generally believed that the surface methyl species formed by chemical adsorption of TMA are sufficiently water-reactive to produce surface hydroxyls. However, recently, there have been several reports on the presence of persistent methyl species in the water exposure step during the Al2O3 ALD process. Here, we performed in situ Fourier-transform infrared spectroscopic analysis to provide evidence that the surface methylated by TMA retains the water-stable methyl species on the surface after exposure to water. The C–H stretching mode of the water-stable methyl species appears at ∼2960 cm–1 with a higher wavenumber compared to that of the reactive methyl species (∼2940 cm–1). Furthermore, the water-stable (persistent) methyl species can be chemically probed with ozone (O3) because ozone reacts with the persistent methyl species to form formate species. During the chemical probing by exposing the persistent methyl species to ozone, the C–H stretching mode of the persistent methyl species disappears at ∼2960 cm–1 and the asymmetric stretching mode of the formate species is observed at ∼1609 cm–1. This reveals that the persistent methyl groups are consumed to form the formate species by the chemical probing with ozone. In addition, we propose a plausible reaction between the surface formate species and TMA, which may explain the observations made by the in situ FTIR investigation.