X-ray photoelectron spectroscopic and annealing studies of ceramic ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4+\mathrm{\ensuremath{\delta}}}$ prepared by high-temperature and high-pressure oxygen enrichment have identified a superoxide as the predominant oxygen species (\ensuremath{\delta}\ensuremath{\approxeq}0.13) correlated with a superconducting transition at ${T}_{c}$\ensuremath{\approxeq}30 K.
