An apparatus for obtaining scanning tunneling microscopy (STM) images at the mercury/air interface is described. The mercury/air interface is studied as a function of adsorbate surface pressure using a mercury sessile drop that can be expanded and contracted using a digitally controlled microliter syringe. The added feature of this arrangement is the ability to control precisely the volume and, the surface area of the sessile mercury drop. The drop then represents a micro-Langmuir-Blodgett experiment in which thin films of adsorbates can be examined in various stages of compression. The macroscopic phase transitions of an octanethiol surface film during the compression/expansion regime are observed visually, and STM images of the mercury/air interface are obtained before and after thiol deposition.