Small-molecule inhibitors are promising for achieving area-selective atomic layer deposition (ALD) due to their excellent compatibility with industrial processes. In this work, we report on growth inhibition during ALD of Al 2 O 3 on a SiO 2 surface functionalized with small-molecule aminosilane inhibitors. The SiO 2 surface was prefunctionalized with bis(dimethylamino)dimethylsilane (BDMADMS) and (N,N-dimethylamino)trimethylsilane (DMATMS) through solution and the vapor phase. ALD of Al 2 O 3 using dimethylaluminum isopropoxide (DMAI) and H 2 O was performed on these functionalized SiO 2 surfaces. Our in situ four-wavelength ellipsometry measurements show superior growth inhibition when using BDMADMS and DMATMS in sequence over just using BDMADMS or DMATMS. Vapor phase functionalization provided a growth delay of ∼30 ALD cycles, which was similar to solution-based functionalization. Using in situ attenuated total reflection Fourier transform infrared spectroscopy, we show that the interaction of DMAI with SiO 2 surfaces leads to pronounced changes in absorbance for the Si−O−Si phonon mode without any detectable DMAI absorbed on the SiO 2 surface. Detailed analysis of the infrared spectra revealed that the decrease in absorbance was likely caused by the coordination of Al in DMAI to O atoms in surface Si−O−Si bonds without the breaking the Si−O−Si bonds. Finally, we postulate that a minimal amount of DMAI remains adsorbed on surface Si−O−Si bonds even after purging, which can initiate ALD of Al 2 O 3 on functionalized SiO 2 : this highlights the need for higher surface coverage for enhanced steric blocking.