The reactivity of peroxide dianion O-2(2-) has been scarcely explored in organic media due to the lack of soluble sources of this reduced oxygen species. We now report the finding that the encapsulated peroxide cryptate, [O-2 subset of mBDCA-5t-H-6](2-) (1), reacts with carbon monoxide in organic solvents at 40 C to cleanly form an encapsulated carbonate. Characterization of the resulting hexacarboxamide carbonate cryptate by single crystal X-ray diffraction reveals that carbonate dianion forms nine complementary hydrogen bonds with the hexacarboxamide cryptand, [CO3 subset of mBDCA-5t-H-6](2-) (2), a conclusion that is supported by spectroscopic data. Labeling studies and O-17 solid-state NMR data confirm that two-thirds of the oxygen atoms in the encapsulated carbonate derive from peroxide dianion, while the carbon is derived from CO. Further evidence for the formation of a carbonate cryptate was obtained by three methods of independent synthesis: treatment of (i) free cryptand with K2CO3; (ii) monodeprotonated cryptand with PPN[HCO3]; and (iii) free cryptand with TBA[OH] and atmospheric CO2. This work demonstrates CO oxidation mediated by a hydrogen-bonding anion receptor, constituting an alternative to transition-metal catalysis.