Evidence of clay migration from the core to the surface of poly(propylene)/montmofillonite nanocomposites is provided. A three- to fivefold increase in the clay concentration of the surface is obtained during isothermal heating in oxidative atmosphere. The mechanism of migration is investigated by means of attenuated total reflectance Fourier transform infrared, spectrometry. It is shown that oxygen plays a fundamental role in the migration mechanism.