We present an experimental and theoretical investigation into time- vs. frequency-domain femtosecond sum frequency spectroscopy at the metal-liquid interface. Although frequency and time-domain measurements are theoretically equivalent it is demonstrated here experimentally that the two approaches are sensitive to different physical aspects of the system and provide complementary information. Time-domain measurements are demonstrated to be more clearly influenced by the inhomogeneity of adsorption sites, since the decay of the vibrational polarization can be mapped directly. A generalization of existing models allows for the simultaneous description of both frequency and time-domain measurements. (C) 2003 Elsevier Science B.V. All rights reserved.