The vibrational relaxation of several diatomic molecules at the surface of liquid water is studied using classical molecular-dynamics computer simulations and compared with the same process in the bulk liquids. Both nonequilibrium classical trajectory calculations and equilibrium force autocorrleation functions are used to elucidate the factors that influence vibrational energy relaxation at the liquid surface region. We find that in general vibrational relaxation rates at interfaces are slower than in the bulk due to reduced friction. However, the degree of the slowing-down effect depends on the contribution of electrostatic forces and is correlated with the structure of the first solvation shell.