Interfaces are important for many processes in chemistry, physics, and biology. Crucial to their properties are the details of the three-dimensional structure of the participating (macro)molecules. Vibrational Sum Frequency Generation (VSFG) is a tool specifically suited to probing the first few atomic layers of an interface. Traditionally, interfaces are probed by mapping localized vibrational modes. Here, we show that the three-dimensional structure of large interfacial biomolecules can be probed by measuring delocalized vibrational backbone modes, which appear to be extremely sensitive to changes in the skeletal structure. We demonstrate that for three different films of chemically identical poly(lactic acid) polymer, we can observe dramatic changes in the three-dimensional arrangement of the surface molecular backbones. This type of information could not be obtained from probing only localized group modes. © 2008 American Chemical Society.