The use of scanning tunneling microscopy (STM) to characterize chemical sensors is reviewed and the first investigation of the operational mechanism of an active chemical sensor is reported. It is demonstrated that STM can be used to understand the mechanistic aspects of a chemical sensor under actual operating conditions for the detection of toxic mercury vapor by thin gold-film sensors. The operating principles of STM and the effects of surface inhomogeneities are discussed. The STM characterization of the gold-film sensor structure, mercury adsorbate structure and dynamics, sensor poisoning as well as the relationship of these results to the electrical properties of the sensor are presented. The potential future contributions of STM to chemical sensor research are also considered.