Scanning tunneling spectroscopy of hexanethiolate-protected gold nanoparticles was studied in the presence of volatile organic vapors. The particles were immobilized onto a Au(1 1 1) surface that was modified with a hexanethiol self-assembled monolayer. It was found that particle dimensions played a critical role in dictating the STS behaviors in response to exposure to organic vapors, with the optimal core diameter around 4.9 nm. This was interpreted on the basis of solvent penetration into the nanoparticle/self-assembled monolayer interface and the resulting manipulation of nanojunction capacitance and resistance. These fundamental insights may be exploited as a sensing mechanism for chemical vapor detection. (C) 2008 Elsevier B. V. All rights reserved.