(Di-methyl-phosphor-yl)methanaminium hydrogen oxalate-oxalic acid (2/1).

The reaction of (di-methyl-phosphor-yl)methanamine (dpma) with oxalic acid in ethanol yielded the title solvated salt, C3H11NOP(+)·C2HO4 (-)·0.5C2H2O4. Its asymmetric unit consists of one dpmaH(+) cation, one hydrogen oxalate anion and a half-mol-ecule of oxalic acid located around a twofold rotation axis. The H atom of the hydrogen oxalate anion is statistically disordered over two positions that are trans to each other. The hydrogen oxalate monoanion is not planar (bend angle ∼16°) whereas the oxalic acid molecule shows a significantly smaller bend angle (∼7°). In the crystal, the components are connected by strong O-H⋯O and much weaker N-H⋯O hydrogen bonds, leading to the formation of layers extending parallel to (001). The structure was refined from a racemically twinned crystal with twin components in an approximate 1:1 ratio.