Time-of-flight secondary ion mass spectroscopy has proven to be a very powerful tool in the study of the interaction of metal atoms with organic thin films since analysis of the emitted molecular cluster ions provide clear information about the presence of chemical reactions, metal penetration and metal nucleation. In this work, this approach is employed to examine the behavior of K atoms evaporated onto -S(CH2) 15CH3, -S(CH2) 15CO2H and -S(CH2)(15)CO2CH3 self-assembled monolayers on Au substrates. On the -CH3 surface, no chemical reaction is observed, and the sticking probability of K atoms is quite low. However, we find that K atoms react with -CO2H and -CO2CH3 groups, forming -CO2K moieties at the vacuum interface, but do not react with the -(CH2)(n)- backbone. Additional exposure of K to the system results in K remaining at the vacuum interface, not penetrating through the organic layer. The results imply that the CO2K may act as a possible buffer layer to prevent chemical reactions when other types of metal atoms are employed. (C) 2004 Elsevier B.V. All rights reserved.