Thermal and UV degradation of several common space-grade polyimides and silicones was studied in situ using electron spin resonance (ESR) spectroscopy. With this method paramagnetic species, present in a sample, can be detected. Such species include radicals, transition metal ions and colour centres. From their various ESR signals, one may obtain concentrations and, depending on the material, also molecular structures. In the present case, the radicals formed during thermal or UV degradation of films of the polyimides Kapton HN and Upilex S as well as of bulk samples of the DC 93-500 and Elastosil S 690 silicones were studied. By recording subsequent spectra at a sufficient rate, the time dependence of the radical concentration could be followed, allowing more detailed study of the kinetics of the degradation process. The thermal degradation of Kapton HN was found to consist of two main processes, one being a radical termination and the other the actual degradation process. UV degradation experiments showed that the polyimides degrade at a much faster rate than the silicones.