Band offsets of the Al2 O3 /GaSb system and various surface passivation treatments of the GaSb substrate by HCl, NH4 OH, and (NH4)2 S solutions were investigated by x-ray photoelectron spectroscopy. The extracted conduction and valence band offsets values of Al2 O3 relative to GaSb are 2.4±0.1 eV and 3.4±0.2 eV, respectively. The presence of Ga-O and Sb-O bonds was detected after NH4 OH surface treatment. In contrast, (NH 4)2 S and HCl solutions inhibit the Sb oxide formation. The lowest amount of Ga-O bands was obtained for (NH4)2 S passivation. These results correlate with capacitance-voltage (C-V) measurements of Pd-Au/ Al2 O3 /GaSb stacks which yielded the best characteristics for the S-based passivation.