FTi.r. studies of the photooxidation of aromatic diisocyanate (MDI) based polyurethanes are presented. The results obtained show that the photochemical evolution of the aromatic moieties involves a dual mechanism : one is a photo-Fries type reaction whereas the other results in an induced oxidation of the central methylene group of MDI. Direct evidences that these reactions occur are obtained on the basis of the modification of the spectral features resulting from both the rearrangement of urethane functions and the formation of oxidized groups.