We calculated the ionization potentials for 34 CsM molecules (where M is the element of the periodic table) by using SIESTA code, on the basis of the density functional theory. The calculated ionization potentials of CsM molecules were found essentially less than the ionization potentials of related atoms. Experimental SIMS relative sensitivity factors were found for the CsM+ cluster ions by using implanted standards. Exponential dependence between the relative sensitivity factors of CsM+ cluster ions and their ionization potentials was obtained. This lets us suggest that there is a direct emission of the CsM molecules during the cesium ion beam sputtering process; ionization of the CsM molecules occurs at the emitted surface because of the same mechanism as for sputtered atoms. Copyright © 2012 John Wiley & Sons, Ltd.