Electron paramagnetic resonance (EPR) measurements were performed on poly-epsilon-caprolactone (PCL) films at different stages of the postplasma-grafting process. PCL films prepared by solvent casting (SC) or (ESP) yield very similar EPR spectra after Ar-plasma treatment and subsequent exposure to air, but the EPR signal is much stronger in the PCL-ESP films. The free radicals appear to be mainly, and possibly exclusively, oxygen centered. The radicals generated by UV irradiation in PCL-ESP films were studied in situ with EPR, using a UV-LED (lambda = (285 +/- 5) nm). Their EPR spectrum is distinctly different from the plasma-induced signal, indicative of carbon-centered radicals, and appears to be independent of the plasma pretreatment. UV-induced homolytic splitting of (hydro) peroxide bonds was not observed. Both the plasma- and UV-induced radicals decay at room temperature (RT), even in an inert atmosphere. This study demonstrates the potential of electrospun films and UV-LEDs for the study of plasma- and UV-generated free radicals with EPR in polyesters, and raises questions with respect to the validity of some generally accepted molecular mechanisms underpinning the postplasma grafting technique for polyesters. (C) 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 50: 2142-2149, 2012