In the title hydrated mol-ecular salt, C4H12N(+)·C2HO4 (-)·0.5H2O, the O atom of the water mol-ecule lies on a crystallographic twofold axis. The dihedral angle between the CO2 and CO2H planes of the anion is 18.47 (8)°. In the crystal, the anions are connected to each other by strong near-linear O-H⋯O hydrogen bonds. The water mol-ecules are located between the chains of anions and iso-butyl-amine cations; their O atoms participate as donors and acceptors, respectively, in O-H⋯O and N-H⋯O hydrogen bonds, which form channels (dimensions = 4.615 and 3.387 Å) arranged parallel to [010].
Keywords: crystal structure; hydrated molecular salt; hydrogen bonding; isobutylammonium hydrogen oxalate hemihydrate; materials engineering.