In this paper we study the photo-oxidation phenomenon in the structure of a PPV derivative, the poly[(2-methoxy-5-hexyloxy)-p-phenylenevinylene]-1 - (MH-PPV), using ellipsometry and infrared spectroscopy techniques. Ellipsometric measurements were carried out in MH-PPV films previously exposed to light irradiation in air. It was observed a variation of both real and imaginary components of the refractive index. These variations are related with the replacement of the vinyl C=C bounds by the carbonyl C=O bounds, as confirmed by the infrared absorption measurements.