This work focuses on the separation and quantification of fixed bulk oxide charge, fixed charge at the dielectric–semiconductor interface and interface state charge components in the Pt/Al2O3/In0.53Ga0.47As metal-oxide-semiconductor (MOS) system. The availability of atomic layer deposited Al2O3 dielectrics over n- and p-type In0.53Ga0.47As with a range of well-controlled thickness values opens up an experimental route for the determination of the interface state density (Dit) independently of the total fixed oxide charge using capacitance–voltage measurements taken at 1 MHz and −50°C. Low temperature forming gas annealing (350°C) significantly reduces the amount of fixed charge. The interface fixed charge is reduced from ∼ −8.5 × 1012 cm−2 preanneal to ∼ −7.4 × 1011 cm−2 postanneal and the bulk oxide charge is reduced from ∼1.4 × 1019 cm−3 preanneal to ∼5 × 1018 cm−3 postanneal. The forming gas anneal also has a significant effect on the interface state charge, reducing its density from 1.3 × 1013cm−2 preanneal to 4 × 1012 cm−2 postanneal.