Two derivatives of the well-known iron chelator, (E)-N′-(2-hydroxybenzylidene)isonicotinohydrazide (SIH), substituted in the 5-position of the 2-hydroxybenzene ring by a methyl and a fluorine group viz. (E)-N′-(2-hydroxy-5-methylbenzylidene)isonicotinohydrazide, C14H13N3O2, (I), and (E)-N′-(5-fluoro-2-hydroxybenzylidene)isonicotinohydrazide, C13H10FN3O2, (II), have been prepared and characterized by single-crystal X-ray diffraction, 1H NMR and mass spectrometry. The molecules of both compounds deviate slightly from planarity [r.m.s. deviations are 0.145 and 0.110 Å for (I) and (II), respectively] and adopt an E conformation with respect to the double bond of the hydrazone bridge. In each molecule, there is an intramolecular O—H...N hydrogen bond forming an S(6) ring motif. The dihedral angles between the mean planes of the isonicotinoyl ring and the cresol ring in (I) or the fluorophenol ring in (II) are 10.49 (6) and 9.43 (6)°, respectively. In the crystals of both compounds, zigzag chains are formed via N—H...N hydrogen bonds, in the [10-1] direction for (I) and [010] for (II). In (I), the chains are linked by weak C—H...π and π–π stacking interactions [centroid-to-centroid distances = 3.6783 (8) Å; inter-planar angle = 10.94 (5)°], leading to the formation of a three-dimensional supramolecular architecture. In (II), adjacent chains are connected through C—H...O hydrogen bonds to form sheets parallel to (100), which enclose R44(30) ring motifs. The sheets are linked by weak C—H...π and π–π [centroid-to-centroid distance = 3.7147 (8) Å; inter-planar angle = 10.94 (5)°] interactions, forming a three-dimensional supramolecular architecture.