This computational chemistry analysis compares the interactions of H2O and CO2 with zigzag graphene edges, and in particular their adsorption and desorption steps. We provide detailed information regarding the geometric and electronic factors that influence both their adsorption and desorption (of H-2 and CO) processes. Density functional theory results are compared with experimental information to offer heretofore unavailable insights into key aspects of the rate-limiting steps, inhibition phenomena and nanoscale differences in these two reactions. Thus, for example, the orientation and rotation of the adsorbing H2O molecules are elucidated using intrinsic reaction coordinate calculations and molecular orbital analysis, and these results complement recent pulse field gradient NMR measurements and molecular dynamics calculations. Such mechanistic findings reveal the H2O adsorption process to be a slow rotational phenomenon whereas the low-temperature H-2 desorption is geometrically constrained by the presence of contiguous (di)hydrogenated carbon atoms with or without hydroxyl groups. This surface-assisted desorption mechanism is proposed to be responsible for the formation of hexagonal pits during the graphite-H2O reaction. Similar insights were obtained for the graphene-CO2 reaction. Mechanistic schemes are proposed for the desorption products observed in experiments, distinguishing zigzag from armchair sites. (C) 2020 Elsevier Ltd. All rights reserved.