We demonstrate the capability of cluster secondary ion mass spectrometry on characterization of multi-layer ultrathin films assembled by electrostatic adsorption of oppositely charged polyclectrolytes. The samples consisting of layers of polyethylenimine, PEI, and polystyrene sulfonate, PSS, were bombarded with 19 keV (CsI)Cs+ and 21 keV Au3+ projectiles with secondary ions identified by time-of-flight mass spectrometry. The data show the high sensitivity of cluster SIMS to the physicochemical characteristics of the two topmost layers. (C) 2004 Elsevier B.V. All rights reserved.