The direct measurement of the transbilayer movement of 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) in a planar supported lipid bilayer (PSLB) at the fused silica/D2O interface was obtained with sum-frequency generation (SFG) vibrational spectroscopy. The intrinsic sensitivity of SFG to the symmetry of an interface was used to measure the asymmetric distribution of DSPC and perdeuterated DSPC (DSPC-d83) lipids in asymmetrically prepared DSPC/DSPC-d83 PSLBs. Changes in the membrane lipid composition due to exchange between leaflets was monitored by measuring the decay in the CH3 symmetric stretch intensity at 2875 cm-1 with time. The activation energy for transverse motion was determined directly from spectral relaxation measurements at several temperatures and was determined to be 206 +/- 18 kJ/mol. At room temperature (25 degrees C) the half-time of lipid flip-flop was calculated to be approximately 25 days. At 51 degrees C, only 7 degrees C below the main phase-transition temperature of DSPC, the half-time decreases to 25 min. These results have important implications for understanding the transbilayer movement of lipids in biological membranes.