In situ IR spectra of pyridine adsorbed on a polycrystalline gold electrode in 1 M NaF have been measured. An in-plane ring symmetrical vibration due to vertically adsorbed pyridine was observed at around 1605 cm(-1) accompanying a decrease of the same mode band at 1596 cm due to dissolved pyridine. The position and shift of the bands due to vertically adsorbed pyridine were independent of the concentration of pyridine, leading us to deduce that it is caused by a weak Stark effect and/or an effect of the electrode potential on the nonbonding orbital of the nitrogen atom. The vertically adsorbed pyridine was observed at 0.7 V (vs Ag/AgCl), at which potential detection of adsorbed pyridine by an electrochemical method has not been reported. A decrease in an asymmetric band at 1446 cm(-1) due to dissolved pyridine was also observed at more positive potentials.