A new metastable state of O2 has been observed following the charge exchange of fast beams of O2+ with alkali metals. Coupling of the experimental results with ab initio calculations provides strong arguments for the identification of the metastable as the previously unknown 11Δu valence state which adiabatically dissociates to O(1D) + O(1D). The primary relaxation pathway is found to be a spin-orbit induced predissociation by the repulsive 23Δu state for which an HSO = 9 (±1) cm-1 matrix element is determined. Due to accidentally small Franck-Condon factors the υ = 2 level of the 11Δu state is particularly long-lived and evidences extraordinarily large isotope effects with the lifetime for this level being 0.1, 1.8, and 0.5 μs for 32O2, 34O2, and 36O2 respectively. The other low vibrational levels of the 11Δu state are predicted to have lifetimes of ∼5-10 ns, suggesting that high-quality spectra of these states should be obtainable and that chemical reactions of this new state of O2 are possible.