HCO3- reacts with H2O2 yielding monoperoxocarbonate (HCO4-), an oxidizing agent able to epoxidize alkenes. The structures of HCO4- and of parent monoperoxocarbonic acid (HPCA) are not known. However, the structure of the related peroxyformic acid (PFA) has been determined using microwave spectroscopy. We used in this work experimental and theoretical structures of PFA and formic acid, to suggest an approximate experimental structure for HPCA. Using a convenient set of isodesmic reactions, we also calculated the experimentally unknown enthalpies of formation of PFA, HPCA and carbonic acid. These values were further checked using model chemistry methods (G2(MP2), G3 and CBS-APNO). (C) 2009 Elsevier B.V. All rights reserved.