Polarization modulation Fourier transform infrared reflection-absorption spectroscopy (PM FTIRRAS) has been combined with electrochemistry to monitor the potential-induced transformations of a monolayer and a bilayer formed by 4-pentadecylpyridine (C15-4Py), a model amphiphilic compound, at a Au(111) electrode surface. The optical constants for a solution of randomly oriented C15-4Py molecules have been determined and used to calculate the integrated band intensities for a monolayer and bilayer of randomly oriented molecules. The orientation of the hydrophobic and hydrophilic parts of the molecule with respect to the surface normal was determined from the ratio of the intensity of experimental to calculated bands. The CH stretch region of the spectra was used to determine the tilt angle between hydrocarbon chains and the surface normal. The tilt angle varied with potential. The minimum value of the tilt angle was 16degrees for the monolayer and 27degrees for the bilayer. In the monolayer, the headgroup of the C15-4Py molecule lies almost flat on the Au(111) surface. At negative potentials, the angle between the plane of the pyridine ring and the surface normal is 68degrees. As applied potential becomes more positive, the pyridine group stands up gradually and the tilt angle decreases to 63degrees. In the bilayer, the C-2 axis of the pyridine group in the leaflet turned to the electrolyte solution is tilted at similar to70degrees with respect to the normal and this tilt angle changes little with potential. In the leaflet turned to the electrode, the angle between the C-2 axis of the headgroup and the surface normal changes from 64degrees to 54degrees by moving from negative to positive potentials. The PM IRRAS data also show that in the bilayer, the plane of the pyridine group rotates with respect to the C-2 axis when the electrode potential changes. In the monolayer, the tilt angle of the C-2 axis changes but the plane of the pyridine moiety does not rotate with potential.