Sum frequency generation (SFG) vibrational spectroscopy has been successfully applied to study "buried" poly(n-butyl methacrylate) (PBMA)/polystyrene (PS) or deuterated PS (d-PS) interfaces in situ. Orientation information on ester side chain methyl groups of PBMA at these interfaces has been deduced from the SFG spectra and compared to those of PBMA/air and PBMA/water interfaces. Our results indicate that the ester side chain methyl groups have different orientation at different interfaces. These methyl groups tend to stand up on the PBMA surface in air and tend to lie down on the surface upon contacting water. At the PBMA/PS (d-PS) interface, the orientation of ester side chain methyl groups is between the two extremes of the PBMA/air and PBMA/water interfaces. To further prove that the orientation change of methyl groups at the PBMA/PS interface is due to the interaction effect with PS, the "buried" interface between PBMA and a thin layer of polymer with phenyl groups on water has also been investigated.