Phase diagrams of ternary systems : water + oxalic acid + (1-pentanol or + isobutyl acetate or + methyl isobutyl ketone) were obtained at 303.15 K. The system containing 1-pentanol was revised [Faizal et al., J. Chem. Eng. Data 35 (1990) 352-354] in order to show the presence of the dihydrate of oxalic acid, by means of the analysis of the solid phase using FT-IR spectrum, melting point data, and acid-base titration. Experimental results show that all aqueous oxalic acid systems have one region where both acid forms are mixed, and three solid-liquid zones where the solid is the hydrated oxalic acid. The recovery of oxalic acid from aqueous solutions was evaluated analyzing their distribution coefficients, selectivities and distribution curves on a solvent-free basis. None of the three solvents studied here were found suitable for oxalic acid extraction processes, because distribution coefficients are always smaller than 1 and selectivities are not high enough.