Near-field Brillouin scattering spectroscopy is applied to probing interfacial dynamics in condensed matter. We have found that the intensity ratio of an acoustic mode to a central mode differs considerably from that measured for a bulk sample. The difference is more prominent when the penetration depth becomes smaller. From the theoretical considerations, it is found that the uncertainty of momentum due to the discontinuous nature at the interface influences considerably on the intensity of the dispersive modes. (C) 2007 Elsevier B.V. All rights reserved.