Infrared absorption spectra of pyridine- and pyrazine-carboxylic acids have following characteristic absorptions. 1) There are broad absorption bands with peaks at 2450 and 1900 cm-1. The former shifts to around 1900 cm-1 by deuteration but the latter does not. 2) Absorptions for O-H in-plane deformation and C-O stretching vibration are present at 1340∼1300 cm-1 and 1310∼1260 cm-1, and shift respectively to 1360∼1340 cm-1 and 1080∼1020 cm-1 by deuteration. 3) There is no absorption band for O-H out-of-plane deformation and that for [chemical formula] is present at around 700 cm-1. These characteristic absorptions can be explained by the structure (II) with intramolecular hydrogen bonding for nicotinic and isonicotinic acids, and the structures (VI and VII) with intramolecular hydrogen bonding for picolinic and pyrazinic acids possessing a carboxyl in the position alpha to ring nitrogen. The hydrochloride of pyridinecarboxylic acids and 2, 6-disubstituted isonicotinic acid derivatives possess the structure (II) same as that of ordinary carboxylic acids.