A systematic sum frequency generation vibrational spectroscopy study is conducted on the gas-liquid interface of room-temperature ionic liquids. The compounds contain ammonium and pyrrolidinium based cations, to which alkyl substituents of different length and/or functional groups are attached, and they are all combined with the bis-trifluoromethanesulfonimide anion ([imide](-)). The alkyl chain length shows a strong effect on the ordering of the chains at the topmost layer, reaching the maximum order for (C4H9)N+(CH3)(2)(C3H7) and (C6H13)N+C4H8(CH3). There is also evidence of gauche defects for longer alkyl chains, and changes in the spectral features for the shortest ones. The substitution of a carbon atom in the chain by oxygen, and its per-deuteration, provides the means to acquire a more complete description of the surface structure, and an unambiguous assignment of the vibrations detected in the SFG spectra. Finally, a brief comparison between alkylpyrrolidinium, and previously studied alkylimidazolium, cations is also established.