The vibration of the carbonic acid molecule is composed of 12 normal modes. Four modes are stretching the molecule along the O-H bonds, along the C=O double bond, and along the C-O single bond. The stretching mode ν₉(B2) is antisymmetric with respect to the σ_v reflection plane. The stretching modes ν₁(A1), ν₂(A1), ν₄(A1) are symmetric with respect to the σ_v reflection plane. The other 8 modes describe angular deformations. The deformation modes ν₁₀(B2), ν₁₁(B2), ν₁₂(B2), and ν₆(A2) are antisymmetric with respect to the σ_v reflection plane. The deformation modes ν₃(A1), ν₇(B1), ν₈(B1), and ν₅(A1) are symmetric with respect to the σ_v reflection plane.

Carbonic acid is in the C_2v point group with three symmetry elements: C₂ proper rotation axis (black), σ_v' and σ_v'' reflection plane (yellow and cyan).