The thermal degradation of poly(3-nitratomethyl-3-methyloxetane), also called polynimmo, an important high-energy binder used in propellant formulations, has been examined by a variety of spectroscopic methods combined with exhaustive chromatography. Degradation results in the gradual increase in intensity of two main new absorptions around 1729 and 1550 cm(-1) in the solution infrared (i.r.) spectrum of polynimmo. The band at 1729 cm(-1) is attributed to the carbonyl group in a formate ester of which the associated formate proton and carbonyl carbon are clearly visible in the H-1 and C-13 n.m.r. spectra at 8.1 and 162 ppm, respectively, and can be cross-correlated by two-dimensional n.m.r. The absorption at 1550 cm(-1) is attributed to the asymmetric stretch of a nitro group attached to a tertiary carbon. We propose that this nitro species is formed by the recombination of NO2, following the loss of NO2 and subsequent elimination of CH2O from the polynimmo side-chains, with the resulting carbon radical. The assignment of this nitroalkane was confirmed by electrospray mass spectrometry, (including collision-induced decomposition studies) and n.m.r. spectroscopic characterization of a nearly pure sample of the cyclic tetrameric nitro species isolated from pyrolysed polynimmo using column chromatography. Degradation also results in the development of a strong electron spin resonance absorption attributed to a nitroxide radical produced in a secondary reaction.