The chemisorption of an oxygen adatom on the (0 0 1) surface of a perovskite crystal is examined parametrically. An empirical tight-binding method is used in the calculations. The adatom is assumed to chemisorb in a head-on position to the surface cation and the effects of the following parameters are investigated: the orbital interaction Vp between adatom and surface cation, the difference ΔEd in the self-energies of a surface cation and a bulk one, and the local perturbation ΔW0 on the self-energy of the surface cation that participated in the chemisorption bond. As a result of chemisorption the oxygen adatom shares the delocalized surface states of the perovskite, and in addition to that two peaks appear next to the surface states, one below the surface band being a bonding state and one above it, being an antibonding state. The change in chemisorption energy with the adatom-surface interaction strength has been investigated. © 1994.