A new study may settle a long-standing issue about which water molecules near the surface of an air-water interface have properties different from those of bulk water. The air-water interface covers more than 70% of Earth’s surface, and its characteristics significantly affect water behavior. But the thickness of the layer of surface water molecules affected by air has remained uncertain. Alexander V. Benderskii, currently at the University of Southern California, and coworkers have now analyzed the interface with isotope-dilution methods and heterodyne-detected sum-frequency generation spectroscopy, a technique they developed. The experimental data, together with computer simulations and calculated spectra by James L. Skinner and coworkers at the University of Wisconsin, Madison, suggest that the air-water interface is thin—approximately one molecular layer (Nature, DOI: 10.1038/nature10173). The combined technique could also be used to study other interfaces, such as the water-biomembrane boundary, Benderskii says.