The chemical activity of the heavier POSS homolog, n-butylstannoxane dodecamer cage, in an epoxy matrix containing polopropylene oxide (PPO) chains was studied in detail, especially the long-time development of the effect and its limits in time at different concentrations. A multi-method investigation was carried out, employing spectrometric analysis of chemical composition (NMR, IR), TEM, thermogravimetry, weight loss analysis during isothermal oxidation at 180 degrees C, as well as dynamic-mechanical thermal analysis (DMTA) of intact and of oxidized epoxy/stannoxane hybrids. The PPO segments of the matrix were found to be preferentially degraded by oxidation or by anaerobic pyrolysis, while at the same time these segments are the sites of crosslinldng reactions with stannoxane, which counteract matrix degradation. It was demonstrated, that the cages undergo repeated reactions on the Sn atoms, and remain finely dispersed and well-accessible even after long oxidation times. The repeated reactions also explain the observed very strong stabilizing effect of stannoxane at very low concentrations. Efficient stabilization against the degradation of the mechanical properties was found to require higher but still very low concentrations, than the stabilization against weight loss. Interestingly, certain amounts of stannoxane caused simultaneously enhanced weight losses and a mechanical reinforcement due to a higher amount of radical reactions.