A procedure has been developed to determine the growth kinetics of thin (≪3 nm) oxide films on bare binary alloys from the measured changes of the ellipsometric amplitude-ratio and phase-shift dependent parameters, ψ and Δ, versus wavelength, as function of oxidation time, as recorded by real-time, in situ spectroscopic ellipsometry. The approach has been applied to the dry, thermal oxidation of Mg-based MgAl substrates of low (2.63 at. %) and high (7.31 at. %) bulk Al content at 304 K within the partial pressure of oxygen range of 10-6–10-4 Pa. Various models have been developed to describe the time dependences of the spectra of ψ(λ) and/or Δ(λ) for the initial and subsequent stages of oxidation. It followed that the initial oxide-film growth kinetics can be accurately described by adopting a three-node graded oxide layer using the Maxwell-Garnet effective medium approximation to assess the optical properties of the compositionally inhomogeneous, Al-doped MgO film developing on the MgAl alloy surface. The specific complications that arise in the ellipsometric analysis of the oxidation of binary alloys (as compared to that of pure metals), such as due to the concurrent processes of selective oxidation and (oxidation-induced) chemical segregation, have been discussed.