In this paper, we review the experimental results that were obtained using our high pressure-high temperature STM system (HP/HT STM) for studies of gas adsorption in a broad pressure range. Measurements are carried out in equilibrium with the gas phase. We discovered ordered surface structures of adsorbates that do not exist at low pressures. It appears that small increases in coverage due to increased ambient pressures cause ordering due to repulsive adsorbate-adsorbate interactions. Adsorption isotherms, one of the oldest fields of surface thermodynamics, can be revisited using HP/HT STM to obtain molecular surface structures and surface phase diagrams as the gas pressure is altered.