Electronic structure calculations are used to determine energy barriers to hydrogen radical migration in structure II clathrate small and large cages. Migration of H-radicals through pentagonal and hexagonal faces of small and large cages are considered and energies barriers calculated at the MP2 level with the 6-311++G(d,p) basis set are 61 and 17 kJ mol (1), respectively. Energy barriers (with tunneling corrections) are used to estimate escape rates from the cages and to explain results of recent experiments on the transformation of n-propyl radical in the propane hydrate and the behavior of hydrogen radicals in tetrahydrofuran/H-2 hydrates. (c) 2009 Elsevier B.V. All rights reserved.