Interfacial structures of alkylated polyvinylpyridine (Cn-RVP) brushes with various side chain lengths (n = 2, 6, 12) in dry nitrogen, water vapor, liquid water, and aqueous electrolyte solution were investigated by sum frequency generation (SFG) spectroscopy. The SFG spectra of the polymer brushes in the CH stretching region in dry nitrogen revealed that the conformational order of the side chain depended on the chain length. The conformational order of the polymer brush with hexyl, C6, side chain, was very high, and the side chain was almost in all-trans conformation with the tilt angle of methyl group being ca. 34 with respect to the surface normal. Significant contribution from CH stretching of methylene group was observed in the SFG spectra of the polymer brush with the shorter, ethyl, C2, or longer, decyl, 012, side chains. Relative intensity of CH stretching of methylene group to that of methyl group, that is, degrees of gauche defects, increased significantly upon in contact with water vapor at all the brushes, and essentially no CH stretching peaks were observed when the brushes were in contact with liquid water, showing almost completely disordered conformational structure. These conformation changes were very fast, suggesting that: the changes were related to the orientation change of the side chains. SFG spectra of the brush/water interface in the OH stretching region were dominated by OH stretching vibration of ice-like water. The higher was the electrolyte concentration, the lower was the OH intensity, reflecting the reduction of the double layer thickness in solution of higher electrolyte concentration.