The dispersion of higher‐order localized elastic modes of gold layers on glass has been measured by Brillouin scattering. For layer thicknessesh≥100 A˚ the dispersion of the sound velocities is well described by elastic continuum theory, whereas forh<100 A˚ deviations occur due to island formation. The results yield a reliable, nondestructive method to determine either elastic constants and mass density or the thickness of thin supported metal layers.