The crystal structures of the complexes of oxalic acid with glycyl-l-histidine and l-histidyl-l-alanine were determined. The three crystallographically independent peptide molecules in the complexes have closed conformations. The terminal carboxyl group of the dipeptide and the oxalate ion in the Gly–His complex exhibit unusual ionization states and are connected by a symmetric O- - -O hydrogen bond. The peptide aggregation in the complex is almost identical to that in the corresponding semisuccinate complex and is similar to one of the predicted aggregation patterns for Ala–Ala, demonstrating that dipeptide aggregation is controlled primarily by main-chain interactions and is substantially unaffected by disturbing influences such as those arising from polar side chains, ions and water molecules. The peptide molecules in the highly pseudosymmetric crystals of the His–Ala complex, however, exhibit a hitherto unobserved aggregation pattern. Thus, in spite of the repeated occurrence of a few patterns, the possibility of the existence of new patterns needs to be taken into account.