Atmospheric-pressure plasma-enhanced spatial atomic layer deposition (PE-s-ALD) is an emerging high-throughput technique used to deposit thin films at low temperatures on large-area substrates. The spatial separation of the ALD half-reactions and the use of an atmospheric-pressure plasma in the co-reactant step give rise to a complex surface chemistry which to date is not well understood. In this study, we employed gas-phase infrared spectroscopy and optical emission spectroscopy (OES) to unravel the underlying chemistry of the PE-s-ALD process for Al2O3 films grown at 80 °C using Al(CH3)3 as the precursor and Ar-O2 plasma as the co-reactant. We identified the reaction products generated at various exposure times of the substrate to the precursor. Infrared absorbance spectra show CO, CO2, H2O and CH4 as the main reaction by-products formed from a) combustion-like reactions of the methylated substrate surface with oxygen radicals and O3 species, and b) H2O molecules either residual or formed in the process that give rise to a concurrent latent thermal ALD component. In addition, CH2O and CH3OH were identified as reaction by-products formed either at the substrate surface or in the plasma. The OES spectra confirmed the combustive nature of the PE-s-ALD reactions as shown by the OH and CH emission peaks that appeared during the spatial ALD process while excited O-species are being consumed.