Vibrational sum-frequency spectroscopy (VSFS) studies of a series of HOD/H 2 O/D 2 O mixtures ranging from pure D 2 O to pure H 2 O have been performed at the vapor/water interface. The various concentrations allow an iterative fitting procedure to be applied, resulting in a set of resonant peaks which consistently describe the vibrational modes of water molecules present in the interfacial region. The resonant sum-frequency response from the contributing vibrational modes allows more definitive characterization than in previous studies of the bonding interactions between surface water molecules. Comparison of the resonant spectrum of the vapor/ H 2 O interface with the sum-frequency spectrum obtained at the CCl 4 /H 2 O interface reveals more similarity between the interfacial hydrogen-bonding environments than previously determined. Recent molecular dynamics simulations of VSF spectra of the vapor/H 2 O interface are in good agreement with the experimentally obtained spectra, and give insight into the molecular interactions in the interfacial region, as well as an estimate of the interfacial depth probed.