In 9H-adenine-1,7-diium hemioxalate chloride, C(5)H(7)N(5)(2+).0.5C(2)O(4)(2-).Cl(-), (I), adenine is doubly protonated, while in 7H-adenin-1-ium semioxalate hemi(oxalic acid) monohydrate, C(5)H(6)N(5)(+).C(2)HO(4)(-).0.5C(2)H(2)O(4).H(2)O, (II), adenine and one oxalate anion are both monoprotonated. In (I), the adeninium cation forms R(2)(2)(8) and R(1)(2)(5) hydrogen-bonding motifs with the centrosymmetric oxalate anion, while in (II), the cation forms R(2)(1)(6) and R(1)(2)(5) motifs with the centrosymmetric oxalic acid molecule and R(1)(2)(5)and R(2)(2)(9) motifs with the monoprotonated oxalate anion. Linear hydrogen-bonded trimers are observed in (I) and (II). In both structures, the hydrogen bonds lead to the formation of two-dimensional supramolecular hydrogen-bonded sheets in the crystal packing. The significance of this study lies in the analysis of the interactions occurring via hydrogen bonds and the diversity seen in the supramolecular hydrogen-bonded networks as a result of such interactions.