A general framework about the influence of reactive particles on surface electronic structure and barrier formation at low coverages (ϑ?10−2) is deduced from experimental results obtained under thermodynamic equilibrium conditions on a suitable compound semiconductor ’’model surface.’’ Adsorption isotherms enable a rough classification of weak and strong electronic interactions into physisorption, chemisorption, and surface reaction steps, which depend on temperatures and partial pressures. (1) Covalent bonding of particles on ionic compound semiconductor surfaces, characterized by small partial charges formally attributed to the adsorption complex, may lead to extremely large dipole effects even at very low coverages [e.g.,dipole moments ≳ 15 Debye may formally be attributed to the adsorption complex ZnO (101̄0)/CO2,chemat ϑ<10−2]. Correspondingly, drastic changes are found in surface atom relaxation. Sticking coefficients are close to unity. (2) Ionic bonding, characterized by pronounced band bending, Sticking coefficients are extremely low for ’’acceptor type’’ adsorption.(3) Point defects at compound semiconductor surfaces are, for entropy reasons, thermodynamically stable at high temperatures. We present for the first time quantitative results on equilibrium concentrations of surface point defects [oxygen vacancies at ZnO(101̄0)], which cause surface Fermi level pinning at extremely low coverages (ϑ<10−3).