The trapping of hydrogen atoms generated by gamma-irradiation of different compounds containing the cube-shaped Si8O12-cage was investigated by means of electron spin resonance (ESR). The trapped hydrogen atoms were found to originate from the organic substituents of the cages. It was shown that intermolecular processes are involved in trapping. This was done by means of a crossover experiment in which d(72)-octakis-(trimethylsiloxy)octasilsesquioxane (i.e., d(72)-Q(8)M(8)) and h(72)-Q(8)M(8) were used. Most surprising, the relative yield of trapped hydrogen atoms is considerably increased by radical scavenging additives (e.g., NO, O-2, and I-2) present during gamma-irradiation. In the presence of radical scavengers, the dose dependence of [H-tr(.)] in, for example, octapropyloctasilsesquioxane becomes almost linear, whereas in the absence of any scavengers it reaches a much lower and quasistationary level. Only for octahydridooctasilsesquioxane (HT8) and octamethyloctasilsesquioxane (MeT8) the yields of atomic hydrogen are not noticeably affected by radical scavengers. This is probably because the scavengers cannot enter the crystal lattices of HT8 and MeT8. If radical scavengers are absent, radicals generated from the substituents can be detected at room temperature for long periods of time. Elemental iodine facilitates the trapping of hydrogen atoms even in solutions of silsesquioxanes in cyclohexane. Moreover, there are radiation induced processes, which remove trapped hydrogen atoms from their traps, so the detected concentration appears to be a net effect. Deoxygenated solutions of irradiated specimen advantageously allow the observation of a well-resolved Si-29-superhyper-finestructure (shfs). The comparison of the experimental shfs pattern with the theoretically expected one proves convincingly the encapsulation of hydrogen atoms in intact Si8O12-units. The values of the shf-coupling constants depend on the nature of the substituents attached to silicon and decrease with increasing temperature. The thermal decay process of H-. trapped in the solid; state is not affected by the atmosphere present and follows first-order kinetics. This corresponds with the uniform trap sites. The activation energy for this process is estimated to 109.6 +/- 3.1 kJ/mol (343 less than or equal to T less than or equal to 387 K) in the case of H-.:Q(8)M(8). The satellites of the hydrogen hyperfine transitions were shown to be spin-flip satellites. proposals in order to explain the observed effects are made.