Using electron paramagnetic resonance, researchers have found experimental evidence for a cobalt(IV) species that plays a catalytic role in water oxidation, a mechanistic insight that might aid energy research (J. Am. Chem. Soc., DOI: 10.1021/ja1013344). Electrochemically splitting water into hydrogen and oxygen is a promising direction for capturing solar energy, and several research teams are developing cobalt catalysts for the difficult water oxidation part of the process. Although Co(IV) intermediates have been implicated in the catalytic cycle, no spectroscopic evidence for the species is available, leaving the pathway uncertain. William H. Casey and R. David Britt of the University of California, Davis; Daniel G. Nocera of Massachusetts Institute of Technology; and coworkers report using EPR to probe water oxidation at neutral pH mediated by a heterogeneous cobalt catalyst previously developed in Nocera’s lab. Their measurements revealed the signatures of Co(IV) and other cobalt species and suggested that Co(IV) had been formed over the course of the water oxidation. The team next plans to employ different EPR techniques to identify important oxygen intermediates, elucidate how Co(IV) charge is distributed, and determine how cobalt is coordinated to its ligands in the catalyst.