The thickness and the refractive index of the surface layer that grows naturally on In0.7Ga0.3AsxP1-x (x = 0.62 and 1.0) thin films exposed to clean room air was measured. Measurements were performed by phase-modulated spectroscopic ellipsometry as a function of time in the range of 100 min-10 months and also as a function of energy For higher accuracy the dielectric function of as-grown In0.7Ga0.3AsxP1-x thin films which was measured in-house in a nitrogen atmosphere was used in data analysis. Within the time range of the experiment the thickness of the surface layer was not saturated. It first increased logarithmically with its knee at approximately I week after exposure to air, and then increased linearly. The refractive index stayed similar to that of the native oxide on GaAs up to approximately 6 months and decreased thereafter. A model in which the surface layer consists of an In0.7Ga0.3As0.62P0.38 oxide layer and an overlying layer of lower refractive index explains the results.