Poly(ethylene terephthalate) was exposed to radiation from different kinds of low-pressure plasmas in an oxygen atmosphere. The lower wavelength limit of the spectrum investigated, lambda = 112 nm, is the cut-off of magnesium fluoride used for separating the specimen chamber from the plasma light source. The total surface oxygen concentration, and the formation of hydroxyl, carbonyl, and carboxyl groups were evaluated from XPS measurements in combination with chemical derivatizations, and their dependences on the radiation spectrum and the oxygen pressure in the sample chamber have been investigated.