We formulated a simple mathematical model of an adsorbed xenon propellant storage system, composed of (i) a stress model for the thin-walled, spherical pressure vessel coupled with (ii) a Langmuir adsorption model to describe the adsorbed xenon density in the porous material. We used the model to find the optimum storage pressure that minimizes the mass of the storage materials (composed of (i) the metal comprising the walls of the pressure vessel and (ii) the adsorbent material) required to carry the xenon onboard the spacecraft. We derived eqns. 10 and 11, interpretable expressions for the optimum storage pressure and associated minimum tankage fraction, revealing how properties of the materials (both the metal and the adsorbent) affect the performance of the adsorbed xenon storage system. Fig. compares the performance of several adsorbent materials for Xe propellant storage, with bulk Xe storage as a baseline.