A size-selected Ar gas cluster ion beam (GCIB) is applied to secondary ion mass spectrometry (SIMS) of a polystyrene (PS) thin film, a 1,4-didodecylbenzene (DDB) thin film, and an ITO glass sample. All three samples are contaminated by poly(dimethylsiloxane) (PDMS) on the surface. Compared with the Ar+ SIMS spectra, the fragments in the PS and DDB SIMS spectra for Ar1550+, including siloxane, are enhanced more than ~100-fold, whereas the hydrocarbon fragments are enhanced 10- to 20-fold. The shallow volume sputtering by GCIB is responsible for the preferential enhancement of the surface contaminants. The intensities of the fragment PDMS ions and the intact DDB ion increase, but the hydrocarbon fragment ions from PS and DDB decrease as the energy per atom (Eatom) of the Ar cluster projectile decreases from ~8 to ~4 eV. These results indicate that tuning Eatom can maximize the surface sensitivity and the sensitivity for intact ions. Copyright © 2012 John Wiley & Sons, Ltd.