Like a hermit crab occupying an abandoned seashell, the transient peroxide dianion (O22–) has been enticed by researchers at Massachusetts Institute of Technology to take up residence in a hydrogen-bonding macromolecule (Science, DOI: 10.1126/science.1212678). Generating stable, soluble sources of O22– has historically been a challenge in dioxygen chemistry. Reduction of O2 to O22– is typically carried out in chemical and biochemical oxidation processes by transition-metal complexes. Nazario Lopez, Christopher C. Cummins, Daniel G. Nocera, and coworkers hypothesized that O22– could be stabilized without a metal if it were surrounded by a hydrogen-bonding environment. The team found that the hexacarboxamide cryptand fit the bill, serving as an anion receptor for O22–. The researchers prepared gram quantities of cryptand-encapsulated O22– either by the disproportionation of KO2 or by the reduction of O2 by cobaltocene. The dianion is stabilized by a combination of six strong hydrogen bonds to the cryptand’s six amide hydrogen atoms (shown) and by six weak hydrogen bonds to the cryptand’s three aryl hydrogen atoms (not shown). The researchers used electrochemical experiments and simulations to understand how the mechanism of O2 reduction at an electrode surface is profoundly altered in the presence of the cryptand.