1:1 Cocrystals of oxalic and fumaric acid with 1,4-diazanaphthalene (quinoxaline) and 2,3-diazanaphthalene (phthalazine) were prepd. 1,5-Diazanaphthalene (naphthyridine) formed a 1:1 cocrystal only with oxalic acid. x-ray structural analyses are reported. Crystallog. data are given. The influence of the position of the two N atoms in these diazanaphthalenes on the crystal packing is analyzed. The gross crystal structure is governed by strong OH...N H bonds. The fine-tuning of the packing of the cocrystals of quinoxaline with oxalic and fumaric acid occurs via CH...O H bonds. The stabilization by the CH...O H bonds leads to a planar sheetlike structure in the cocrystal with fumaric acid, in contrast to the cocrystal with oxalic acid where the optimization of the CH...O H bonds results in a nonplanar bent structure. The geometrical arrangement of the N atoms in 1- and 5-positions in naphthyridine allows a planar sheetlike structure in the cocrystal with oxalic acid. In the case of phthalazine complete proton transfer from one carboxy group of oxalic and fumaric acid to a N atom of the heterocycle, generating ionic interactions is obsd. The H atom of the 2nd carboxy group of the acids is involved in a OH...O:C H bond.