Despite the availability of several mature structure determination techniques for bulk proteins, determination of structural and orientational information of interfacial proteins, e.g., in cell membranes or on biomaterial surfaces, remains a difficult problem. We combine sum frequency generation (SFG) vibrational spectroscopy with attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR) to investigate the orientation of alpha-helical peptides reconstituted in substrate supported lipid bilayers. Melittin was chosen as a model for alpha-helical peptides, and its orientation when interacting with a supported 1,2-dipalmitoyl-sn-glycero-3-phosphoglycerol (DPPG) bilayer has been examined. Through polarization analysis using amide I signals obtained from both SFG and ATR-FTIR measurements, the orientation distribution of melittin inside a DPPG bilayer was deduced using several trial distribution functions. Melittin was modeled as either an ideal helix or a helix with a bent structure. It was found that a simple distribution function such as a delta-distribution or a Gaussian distribution was not adequate to describe the melittin orientation distribution inside a DPPG bilayer. Instead, two populations of melittin, corresponding to two melittin-bilayer association states, could be used to interpret the experimentally observed result. The method employed in this study demonstrates the feasibility of acquiring a more accurate orientation distribution of peptides/proteins in situ using a combination of vibrational spectroscopic techniques without exogenous labeling.