The initial growth during the atomic-layer deposition (ALD) of Al2O3 using trimethylaluminum (TMA) and water was studied on two starting surfaces: SiO2 and −H-terminated Si(111) [H/Si(111)]. In situ spectroscopy ellipsometry (SE) showed virtually immediate growth of Al2O3 on both surfaces, although for H/Si(111) a reduced growth-per-cycle was observed in the initial 20 cycles. The underlying surface chemistry during the initial cycles of ALD was monitored with in situ broadband sum-frequency generation (BB-SFG) spectroscopy. For the SiO2 surface, the −CH3 surface groups were followed revealing that only the first TMA half-cycle deviates from the steady-growth regime. The reaction cross section of the initial TMA half-cycle (σTMA = 2.0 ± 0.2 × 10–18 cm2) was a factor of 3 lower than the cross section of the TMA half-cycle during the steady-growth regime of ALD (σTMA = 6.5 ± 0.6 × 10–18 cm2). All H2O half-cycles, including the first, showed steady-growth behavior with a corresponding reaction cross section (σH2O = 4.0 ± 0.4 × 10–20 cm2). Therefore, only the first ALD cycle was affected by initial growth effects on the SiO2 starting surface, in line with the SE data. For the H/Si(111) surface, the Si–H groups were monitored with BB-SFG spectroscopy, revealing a reaction cross section of σTMA = 3.1 ± 0.3 × 10–18 cm2 for the first TMA half-cycle on H/Si(111); a factor two lower than that during the steady regime of Al2O3. These results demonstrate that the chemistry during the initial growth regime of Al2O3 ALD on SiO2 and H/Si(111) shows subtle but measurable differences compared to the steady-growth regime.