Quinoline-2-carboxylic acid (quinaldic acid), C10H7NO2, crystallizes in the monoclinic space group P2(1)/c, a = 9.724(1) Angstrom, b = 5.937(1) Angstrom, c = 27.545(2) Angstrom, beta = 90.15(1)degrees, z = 4, and occurs in the crystal in two tautomeric forms: the neutral molecule and the zwitterion, with molecular ratio 1:1. The compound can be named quinoline-2-carboxylic acid . quinolinium-2-carboxylate. The crystal structure is built up of tautomeric pairs consisting of C9H6NCOOH and C9H6NH+COO- moieties, held up together by hydrogen bonds.