Two different polymorphs of carbonic acid, α-and β-H 2 CO 3 , were identified and characterized using infrared spectroscopy (FT-IR) previously. Our attempts to determine the crystal structures of these two polymorphs using powder and thin-film X-ray diffraction techniques have failed so far. Here, we report the Raman spectrum of the α-polymorph, compare it with its FT-IR spectrum and present band assignments in line with our work on the β-polymorph [Angew. Chem. Int. Ed. 48 (2009) 2690–2694]. The Raman spectra also contain information in the wavenumber range ∼90–400 cm −1 , which was not accessible by FT-IR spectroscopy in the previous work. While the α-polymorph shows Raman and IR bands at similar positions over the whole accessible range, the rule of mutual exclusion is obeyed for the β-polymorph. This suggests that there is a center of inversion in the basic building block of β-H 2 CO 3 whereas there is none in α-H 2 CO 3 . Thus, as the basic motif in the crystal structure we suggest the cyclic carbonic acid dimer containing a center of inversion in case of β-H 2 CO 3 and a catemer chain or a sheet-like structure based on carbonic acid dimers not containing a center of inversion in case of α-H 2 CO 3 . This hypothesis is strengthened when comparing Raman active lattice modes at <400 cm −1 with the calculated Raman spectra for different dimers. In particular, the intense band at 192 cm −1 in β-H 2 CO 3 can be explained by the inter-dimer stretching mode of the centrosymmetric RC(OHO) 2 CR entity with R OH. The same entity can be found in gas-phase formic acid (R H) and in β-oxalic acid (R COOH) and produces an intense Raman active band at a very similar wavenumber. The absence of this band in α-H 2 CO 3 confirms that the difference to β-H 2 CO 3 is found in the local coordination environment and/or monomer conformation rather than on the long range.