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The 1,4-butane diammonium (putrescine) ion co-crystalizes with propanedioic acid (malonic acid) monoanions in space group Pcab (1,4-butane diammonium hydrogen propanedioate, (C4H14N2)2+.2(C3H3O4)-). One of the carboxylate moieties of malonic acid is protonated. The asymmetric unit of the crystal contains one molecule of malonic acid and half a molecule of putrescine. All three H atoms of the putrescine amino groups participate in hydrogen bonding.