The 1:1 co-crystal N′-[(2-methyl­phen­yl)methyl­idene]pyridine-4-carbohydrazide–benzoic acid (1/1) formed unexpectedly after autoxidation of benzaldehyde during the slow evaporation process of a solution of isoniazid in benzaldehyde. The original intent of the synthesis was to modify isoniazid with benzaldehyde and crystallize the product in order to improve efficacy against Mycobacteria species, but benzoic acid formed spontaneously and co-crystallized with the intended product, N′-benzyl­idene­pyridine-4-carbohydrazide.