Due to high electron mobility, low gate leakage current and the potential for positive threshold, GaAs-based III-V MOSFETs are of interest for high-speed circuit applications. The quality of gate dielectrics is crucial for III-V MOSFET devices. The thermal grown InAlP oxide (InAlP-ox) is promising in terms of its low processing cost, excellent insulating property, as well as an inward growth mechanism, which provides a cleaner oxide-semiconductor channel interface. In the fabrication of GaAs MOSFET devices utilizing thin InAlP native oxides, the oxidation process must be accurately understood and controlled. In this work, variable angle spectroscopic ellipsometry (VASE) is utilized to both characterize InAlP oxidation kinetics and to accurately determine the final thickness of the oxide. To accurately characterize the oxidation of thin (5 nm) InAlP layers used in a MOSFET structure (to yield an ~8 nm thick native oxide dielectric layer), accurate optical constants of each material and an accurate model describing the oxidation dynamics of InAlP are required. The optical constants of InAlP and InGaP (lattice matched to GaAs) have been determined by a multi-sample VASE analysis (1.45 to 5.45 eV, 300 K) of 100 and 200 nm thick epilayers. The optical constants of InAlP-ox are determined by analysis of a 30 nm InAlP epilayer on GaAs, fully oxidized at 440°C for 85 min. The InAlP-ox optical constants are described by Tauc-Lorentz (TL) oscillator terms, and the optical constants of InAlP and InGaP are described by Hersinger-Johs parameterized semiconductor oscillator functions. Comparisons are made between the optical constants of the fully-oxidized and “over-oxidized” InAlP-ox samples (oxidation times of 128 and 170 min). Upon full oxidation, no significant thickness change of the InAlP-ox layer is observed. However, with increasing oxidation time, the oxide refractive index n~1.6 below its bandgap decreases, and the bandgap increases (from ~3.3 to ~4.5 eV). These results suggest that the oxidation rate of the aluminum component in InAlP is faster than that of the less reactive indium component, and that the indium component continues to oxidize after full oxidation of the aluminum component is reached. Accordingly, a three-layer model (ambient/InAlP-ox/EMA/InAlP/GaAs substrate) is used to test InAlP and InAlP-ox optical constants for a 100 nm InAlP film partially oxidized for 100, 120, 130 and 140 min. The effective medium approximation (EMA) layer describes the incomplete oxidation of the indium component in InAlP-ox, and excellent agreement between thickness values measured by transmission electron microscopy (TEM) imaging and VASE confirms the accuracy of our optical constants. Finally, these models have also been extended to accurately fit the thickness of <10 nm InAlP-ox gate oxides grown directly upon a multi-layer MOSFET heterostructure. The deviation of InAlP-ox thickness results determined by VASE from those determined by TEM is within 4%.