The adsorption geometry of pyridine at the Au(111)/electrolyte interface has been studied for the first time by in situ visible-infrared difference frequency generation (DFG) in the spectral range 900-1150 cm(-1) using the CLIO free electron laser. The pyridine molecule was found to adsorb using the nitrogen lone pair, in an end-on configuration, the molecular plane being tilted. The potential dependence of the DFG spectra showed evidences of a reorientation of the molecule around the potential of zero charge (pzc). At a positively charged interface, the molecule tilts towards the surface normal and undergoes a rotation around its C-2 axis.