Surface passivation of high mobility semiconductors such as InGaAs is a crucial bottleneck towards their integration in metal-oxide-semiconductor devices. The chemical structure and band offsets of InGaAs-Al 2O 3 with different passivations were investigated by x-ray photoelectron spectroscopy. Pre-deposition forming gas plasma treatment is shown to significantly improve the chemistry of S-passivated InGaAs surface, on which the Al 2O 3 is deposited by the molecular atomic deposition technique. Moreover, the change in the surface chemistry was found to correlate with a difference of 0.8 eV in the band offsets at the interface. This may offer insights on Fermi level pinning in such systems.