The bromomethane clathrate hydrate crystallizes into the type I structure with the six large cages and the two small cages both filled with bromomethane guest molecules. Dynamic properties of the guest molecules have been investigated by means of incoherent quasi-elastic neutron scattering (QENS) in temperatures between 50 K and 260 K at atmospheric pressure. These investigations provide direct experimental evidence about the fundamental aspect of rotational motions of the bromomethane molecules. A comprehensive study has been achieved through the analysis of the structure factors (i.e., the spatial extend of the motion) as well as of the QENS broadening (i.e., the characteristic time of the motion). Such analysis reveals the existence of two types of motions: methyl group rotations about the C -Br bond with activation energy of 0.19 +/- 0.08 KJ/mol and whole molecule reorientations thermally activated above 100 K with activation energy of 4.3 +/- 1.2 KJ/mol. At the lowest studied temperature (50 K), the methyl group rotates faster by almost one order of magnitude in the large cage (characteristic time of 1.25 ps) than in the small cage (characteristic time of 10 ps). Such results provide new experimental signatures of the cage occupancy by evidencing different Brownian dynamics for the guest molecules in small cages and those in large cages. (C) 2015 Elsevier B.V. All rights reserved.