The relevant parameters which characterise molecular desorption of O 2 -Ag(001) have been determined under isothermal conditions in the low coverage limit, using the beam-flag method. We find a desorption order n = 1.07 ± 0.10, a desorption energy E d e s = 0.50 ± 0.12 eV and a pre-exponential factor v d e s = 10 1 3 . 3 ± 3 . 5 s - 1 . Such values compare well with the thermal desorption parameters present in literature for molecular oxygen adsorbed on Ag(110). The much lower dissociative adsorption probability of O 2 on Ag(001) is therefore due to the existence of a different dissociation mechanism and not to a difference in the desorption probability compared with Ag(110).