Photoellipsometry, a contactless optical method, was used for the characterization of In0.46Ga0.54P/ n+-GaAs heterostructures exposed to room air. Two samples were studied, each containing an undoped In0.46Ga0.54P layer of thickness L (L=40 and 100 nm) grown on top of a heavily doped (100) GaAs substrate by metal-organic chemical vapor deposition. The measured spectra were analyzed using the Franz-Keldysh theory, with inclusion of broadening effects. The results of our specral analysis indicate that, for each sample investigated, there existed an almost uniform built-in electric field in the top layer. Using the built-in field strength obtained from the spectral analysis and the In0.46Ga0.54P layer thickness, we found that the surface Fermi level for each sample was pinned near mid-band-gap energy of In0.46Ga0.54P, with a value of 0.94–0.95 eV.