Long held to be unstable or metastable in the gas phase, carbonic acid has successfully been produced and identified in its gaseous form in recent decades. Theoretical studies have indicated that isolated carbonic acid in the gas phase may in fact be quite stable, its decomposition attributable to the catalytic effect of water molecules, either present or produced in a chain reaction by an initially slow decomposition. In this study, a previously unreported autocatalytic decomposition route is studied using high-accuracy ab initio quantum chemical methods. Results indicate that a carbonic acid dimer may react and decompose in a single-step, highly concerted reaction. The transition state of this reaction was characterized, and the reaction pathway was found to have significantly lower activation energy than the uncatalyzed decomposition, and comparable or lower energy than the water-catalyzed reaction. The results indicate that gaseous carbonic acid should be unstable even in the absence of water. © 2013 Wiley Periodicals, Inc.