Plasma-assisted atomic layer deposition (ALD) of metal oxide films is increasingly gaining interest, however, the underlying reaction mechanisms have rarely been addressed. In this work, a case study is presented for the plasma-assisted ALD process of Al2O3 based on Al(CH3)3 dosing and O2 plasma exposure. A complementary set of time-resolved in situ diagnostics was employed, including spectroscopic ellipsometry, quartz crystal microbalance, mass spectrometry, and optical emission spectroscopy. The saturation of the Al(CH3)3 adsorption reactions was investigated, as well as the reaction products created during both the precursor dosing and the plasma exposure step. The generality of the observations was cross-checked on a second commercial ALD reactor. The main observations are as follows: (i) during the precursor dosing, the Al(CH3)3 predominantly binds bifunctionally to the surface at 70 °C through a reaction in which H is abstracted from the surface and CH4 is released into the gas phase; (ii) during the plasma exposure, O radicals in the plasma are consumed at the surface by combustionlike reactions with the surface −CH3 ligands, producing mainly H2O, CO2, and CO; (iii) small gas phase densities of CH4 and higher hydrocarbons (C2Hx) are also present during the O2 plasma exposure step indicating complementary surface reactions including a secondary thermal ALD-like reaction by the H2O produced at the surface; (iv) the plasma and its optical emission are strongly affected by the surface reaction products released in the plasma. In the latter respect, optical emission spectroscopy proved to be a valuable tool to study the surface reaction products during the plasma exposure as well as the saturation of the surface reactions. The implications of the experimental observations are addressed and it is discussed that the reaction mechanisms are generic for plasma-assisted ALD processes based on metal organic precursors and O2 plasma as oxidant source.