Saponins are natural amphiphilic surfactants with high surface activity and unique surface properties at the air-water interfaces. We have applied sum frequency generation (SFG) vibrational spectroscopy to study escin, a kind of saponins that is extracted from chestnut (Aesculus hippocastanum), which adsorbed at water surfaces as a function of the solution concentrations, and have characterized the net orientation of the hydrophobic aglycone moieties at the water interfaces by varying the solution concentration of the escin. SFG results of escin adsorbed water surfaces demonstrate that the hydrophobic aglycone moieties show nearly standing-up configuration above the critical micelle concentration (cmc), while its preferred orientation is nearly parallel to the surface below cmc. In addition to the changes in the CH stretching band intensities from the escin adsorbed interfaces, we found that the structural arrangement of the escin molecules at the air-water interface is also a function of solution pH and clarified the correlation between the ionic state of the solution and the structure.