The present study refers to the preparation of isonicotinic acid hydrazide (isoniazid, inh) co-crystals with two α-hydroxycarboxylic acids. The interaction of glycolic acid (H₂ga) or mandelic acid (H₂ma) resulted in the formation of co-crystals or salts of composition (inh)·(H₂ga) (1) and [Hinh]⁺[Hma]^–·(H₂ma) (2) when reacted with isoniazid. An N'-(propan-2-ylidene)isonicotinic hydrazide hemihydrate, (pinh)·1/2(H₂O) (3), was also prepared by condensation of isoniazid with acetone in the presence of glycolic acid. The prepared compounds were well characterized by elemental analysis, and spectroscopic methods, and their three-dimensional molecular structure was determined by single crystal X-ray crystallography.