Data are presented on the insulating properties and capacitance–voltage characteristics of metal–oxide–semiconductor (MOS) device-thickness (below ∼100 nm) native oxides formed by wet thermal oxidation of thin InAlP epilayers lattice matched to GaAs. Low leakage current densities of J=1.4×10-9 A/cm2 and J=8.7×10-11 A/cm2 are observed at an applied field of 1 MV/cm for MOS capacitors fabricated with 17 and 48 nm oxides, respectively. Transmission electron microscopy images show that the In-rich interfacial particles which exist in 110 nm oxides are absent in 17 nm oxide films. Quasistatic capacitance–voltage measurements of MOS capacitors fabricated on both n-type and p-type GaAs show that the InAlP oxide–GaAs interface is sufficiently free of traps to support inversion, indicating an unpinned Fermi level. These data suggest that InAlP native oxides may be a viable insulator for GaAs MOS device applications.