Atomic force microscopy has been used to study the structure and the stability of CH3(CH2)9SH and CH3(CH2)17SH molecules self-assembled on a Au(111) surface as a function of the load applied by the tip. Atomic resolution images taken during a loading and unloading cycle have revealed two sudden transitions corresponding to changes of the periodicity from a (square-root 3 x square-root 3)R30-degrees (due to thiol layers) to a (1 x 1) (due to Au(111)), and back to the (square-root 3 x square-root 3)R30-degrees of thiol layers. These results represent the first observation that under high load, self-assembled n-alkanethiol molecules on Au(111) can be reversibly displaced on the substrate surface by a sharp tip during the scan.