The infrared spectra of 3,4-dihydroxy-3-cyclobutene-1,2-dione (squaric acid) isolated in low-temperature Ar and N 2 matrices are reported. The comparison of the experimental spectra with those theoretically calculated at HF/6-31 + + G * * , MP2/6-31G * * and DFT(B3-LYP)/6-31 + +G * * levels shows that the molecules of the compound isolated in low-temperature matrix adopt the structure with C 2 v symmetry. This structure is different from the one which was found in the crystal. Theoretical calculations of relative energies of C 2 v and C s structures predict the C 2 v conformation to be lower in energy by 8 kJ mol - 1 . Good agreement between the experimental and theoretical spectra allowed for a reliable assignment of the observed IR absorption bands.