Ab initio quantum chemical calculations have been performed for the carbonic acid molecule. Results obtained at the QCISD(T)/6-311++G** level show that formation of gas phase H2CO3 from water and carbon dioxide is endoergic by 43.4 kJ/mol, including zero-point energy corrections. The barrier to this reaction is 217 kJ/mol. The calculations reveal global and local minima associated with rotational isomers of the molecule. Transition states have been characterized for interconversion of the isomers by internal rotation and by intramolecular proton transfer reactions. Calculated vibrational frequencies and infrared band intensities are compared with recent experimental studies of condensed phase carbonic acid prepared by two different synthetic routes.