The twofold ester pyrolysis of carbonates, yielding monomeric carbonic acid, was investigated by computational methods. By comparison with CCSD(T)/cc-pVTZ//B3LYP/ccpVTZ benchmark calculations, the M05-2X method was found to give the most accurate reaction enthalpies among the DFT methods employed. While the first ester pyrolysis of diethyl carbonate, (E)-bis(acetaldoxime) carbonate or di-N,N-dimethylhydroxylamine carbonate will yield the corresponding monocarbonates, these highly labile compounds