We have studied the lattice dynamics of a monolayer antimony-covered InP(110) surface by combining the density-functional-pseudopotential theory and an adiabatic bond-charge model. It is found that the deposition of Sb on InP(110) leads to several characteristic vibrational features. In particular, due to the large mass of the Sb atom, the Rayleigh vibrational mode becomes fully localised near the zone edges, lying well below the acoustic continuum of bulk InP. While the surface vibrational modes are removed from the interior of the acoustic-optical gap region, there are still reasonably large peaks in the density of states in the gap towards the bulk continuum edges. (C) 2002 Elsevier Science B.V. All rights reserved.