IR + visible sum-frequency vibrational spectroscopy has been used to monitor structural changes of a polymer surface in response to alteration of environment. The polymer studied is of the polyurethane type, with poly(dimethylsiloxane) (PDMS) grafted on as end groups. Our data reveal that the polymer surface undergoes a significant restructuring when transferred from air to water. With the polymer exposed to air, the surface spectrum shows that the hydrophobic PDMS segments cover most of the surface. When immersed hr water, the PDMS component retreats from the surface whereas the initially "buried", more hydrophilic part of the polymer chain appears at the surface. The surface structural transformation in response to the environmental change from air to water takes about 25 h at 300 K. The structural change is reversible upon dehydration, but takes only 3 h. The results point to the need to characterize the polymer surface in its working environment in order to correctly describe its surface properties, The consistency between the sum-frequency generation (SFG) data and the contact angle measurement in characterizing the hydrophobicity of the polymer surface demonstrates that SFG is a powerful spectroscopic probe to study and provide insight into how polymer surfaces behave at a molecular level.