Photodesorption of O-2 from size-selected AgnO2 cluster anions with n = 2, 3, 4 and 8 was studied using time-resolved photoelectron spectroscopy (TR-PES). The spectra indicate that relaxations of photo-excited AgnO2 clusters with n = even numbers accompany ultrafast direct O-2 photodesorption. For the odd-numbered cluster Ag3O2-, in contrast, a long-lived excited state is observed, since O-2 might be dissociatively chemisorbed, suppressing direct photodesorption of oxygen. Both, direct desorption and long-lived excited states, have not been observed from adsorbate covered metal surfaces, suggesting unique photochemical properties of such small clusters. (c) 2007 Elsevier B.V. All rights reserved.