The activation of dioxygen is a key step in CO oxidation catalyzed by gold nanoparticles. It is known that small gold cluster anions with even-numbered atoms can molecularly chemisorb O-2 via one-electron transfer from Au-n(-) to O-2, whereas clusters with odd-numbered atoms are inert toward O-2. Here we report spectroscopic evidence of two modes of O-2 activation by the small even-sized Au-n(-) clusters: superoxo and peroxo chemisorption. Photoelectron spectroscopy of O2Au8- revealed two distinct isomers, which can be converted from one to the other depending on the reaction time. Ab initio calculations show that there are two close-lying molecular O-2-chemisorbed isomers for O2Au8-: the lower energy isomer involves a peroxo-type binding of O-2 onto Au-8(-), while the superoxo chemisorption is a slightly higher energy isomer. The computed detachment transitions of the superoxo and peroxo species are in good agreement with the experimental observation. The current work shows that there is a superoxo to peroxo chemisorption transition of O-2 on gold clusters at Au-8(-): O2Aun- (n = 2, 4, 6) involves superoxo binding and n = 10, 12, 14, 18 involves peroxo binding, whereas the superoxo binding re-emerges at n = 20 due to the high symmetry tetrahedral structure of Au-20, which has a very low electron affinity. Hence, the two-dimensional (2D) Au8- is the smallest anionic gold nanoparticle that prefers peroxo binding with O-2. At Au-12(-), although both 2D and 3D isomers coexist in the cluster beam, the 3D isomer prefers the peroxo binding with O-2.