Methyl iodide alloyed with methyl bromide is studied for low methyl bromide concentrations cless than or equal to0.3 by rotational tunneling spectroscopy with neutrons. The appearance of three tunneling bands, their shift with respect to the pure materials and their broadening is explained semiquantitatively on the basis of the crystal structure and global and local changes of interatomic distances based on the r-dependence of intermolecular interactions. Besides the overall reduction of the lattice parameter local free volume around guest molecules is important. A local relaxation of the atomic position by 1.3% towards guest molecules is found. (C) 2004 American Institute of Physics.