Identification of the existence of pyrocarbonate ion C2O52− in molten carbonates exposed to a CO2 atmosphere provides key support for a newly established bi-ionic transport model that explains the mechanisms of high CO2 permeation flux observed in mixed oxide-ion and carbonate-ion conducting (MOCC) membranes containing highly interconnected three dimensional ionic channels. Here we report the first Raman spectroscopic evidence of C2O52− as an active species involved in the CO2-transport process of MOCC membranes exposed to a CO2 atmosphere. The two new broad peaks centered at 1317 cm−1 and 1582 cm−1 are identified as the characteristic frequencies of the C2O52− species. The measured characteristic Raman frequencies of C2O52− are in excellent agreement with the DFT-model consisting of six overlapping individual theoretical bands calculated from Li2C2O5 and Na2C2O5.