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An examination of the crystal structures of di-hydroxy malonic acid 1 (P21/na = 5.384(2), b = 6.268(2), c = 14.354(8) A, β = 103.57(3)°) and N-ethylbenzylammonium di-hydroxy hydrogenmalonate 2 (P212121a = 13.373(2), b = 15.342(4), c = 18.741(4) A,) has highlighted the influence of directional O–H···O interactions in the assembly of hydroxy-substituted dicarboxylic acids. The complementarity of carboxylic and hydroxy groups in 1 creates a very dense 3-D structure (Dc = 1.919 g cm−3) through four O–H···O hydrogen bonds. In 2, the anions form infinite, spiraling chains created by head-to-tail O–H···O hydrogen bonds. Neighbouring chains are crosslinked into a complex 3-D anionic network with channels parallel to a and b. The cations are positioned within these channels and held in place via two N–H···O interactions.