For the first time, the pressure and temperature dependence of a chemical reaction at the solid/solution interface is studied by scanning tunneling microscopy (STM), and thermodynamic data are derived. In particular, the STM is used to study the reversible binding of O-2 with cobalt(II) octaethylporphyrin (CoOEP) supported on highly oriented pyrolytic graphite (HOPG) at the phenyloctane/CoOEP/HOPG interface. The adsorption is shown to follow the Langmuir isotherm with P-1/2(298K) = 3200 Torr. Over the temperature range of 10-40 degrees C, it was found that Delta H-p = -68 +/- 10 kJ/mol and Delta S-p = -297 +/- 30 J/(mol K). The enthalpy and entropy changes are slightly larger than expected based on solution-phase reactions, and possible origins of these differences are discussed. The big surprise here is the presence of any O-2 binding at room temperature, since CoOEP is not expected to bind O-2 in fluid solution. The stability of the bound oxygen is attributed to charge donation from the graphite substrate to the cobalt, thereby stabilizing the polarized Co-O-2 bonding. We report the surface unit cell for CoOEP on HOPG in phenyloctane at 25 degrees C to be A = (1.46 +/- 0.1)n nm, B = (1.36 +/- 0.1)m nm, and alpha = 54 +/- 30 degrees, where n and m are unknown nonzero non-negative integers.