Replacement of self-assembled octanethiolates (OT) on Au with carboxyl-terminated 11-mercaptoundecanoic acid (MUA) in solution was investigated. Lateral force microscope (LFM), which can distinguish the surface areas covered with molecules of different functional groups, was combined with the mu-contact printing technique to study the time dependent replacement reaction. The LFM image of the OT and MUA pattern was deteriorated with the replacement. As the replacement proceeded, the reductive desorption peak of OT continuously shifted towards the position of MUA in the linear-scan voltammogram. Within 50 h in MUA solution, more than 60% of OT molecules on All were replaced by MUA while the reverse process, replacement of MUA with OT, occurred much more slowly. This confirms the higher stability of MUA self-assembled monolayer (SAM) than OT SAM. (c) 2004 Published by Elsevier B.V.