In the title compound, C(8)H(20)N(+)·CHO(3) (-)·3H(2)O, the bicarbon-ate anion, which has a small mean deviation from the plane of 0.0014 Å, fully utilises its three O and one H atom to form various O-H⋯O hydrogen bonds with the three water mol-ecules in the asymmetric unit, generating a hydrogen-bonded layer, which extends along (10[Formula: see text]). The tetra-ethyl-ammonium cations, as the guest species, are accommodated between every two neighboring layers, constructing a sandwich-like structure with an inter-layer distance of 7.28 Å.