We have studied the spatial structure and electronic levels of clusters of the molecule CH3Br to investigate the effects of molecular polarity on these properties. Analysis of the core level photoclectron spectra of initially neutral CH3Br clusters shows that the C 1 s(-1) state has a 30% larger binding energy shift between free molecules and clusters than the Br 3d(-1) state. This difference is attributed to an anti-parallel packing of the molecules induced by the polar character of bromomethane. The results obtained from the analysis of valence cluster spectra also support the proposed structure. (c) 2007 Elsevier B.V. All rights reserved.