The vibrational relaxation of a neutral and an ionic diatomic molecule at the interface between water and carbon tetrachloride is studied using classical molecular dynamics computer simulations as a function of the location along the interface normal. Nonequilibrium classical trajectory calculations and equilibrium force autocorrelation functions are used to determine the role that surface polarity and surface roughness have on the vibrational energy and phase relaxation. It is found that the relaxation rate varies monotonically across the interface as the solute moves from the bulk of one liquid to the bulk of the other liquid. However, while the relaxation of the nonionic solute tracks the average density variation of water (or the CCl4), the ionic solute relaxation is much less sensitive to its surface location. It is demonstrated that this is due to the perturbation of the surface structure by the ionic solute, and that the relaxation rate may be correlated (for both the ionic and nonionic solute) to the variation in the structure of the first solvation shell. (C) 2004 American Institute of Physics.