Sum frequency generation vibrational spectroscopy (SFG) and quartz crystal microbalance with dissipation monitoring (QCM-D) are employed to study the interfacial structure and adsorbed amount of the amino acids L-lysine and L-proline and their corresponding homopeptides, poly-L-lysine and poly-L-proline, at two liquid-solid interfaces. SFG and QCM-D experiments of these molecules are carried out at the interface between phosphate buffered saline at pH 7.4 (PBS buffer) and the hydrophobic deuterated polystyrene (d 8 -PS) surface as well as the interface between PBS buffer and hydrophilic fused silica (SiO 2 ). The SFG spectra of the amino acids studied here are qualitatively similar to their corresponding homopeptides; however, SFG signal from amino acids at the solid/PBS buffer interface is smaller in magnitude relative to their more massive homopeptides at the concentrations studied here. Substantial differences are observed in SFG spectra for each species between the hydrophobic d 8 -PS and the hydrophilic SiO 2 liquid-solid interfaces, suggesting surface-specific different interfacial ordering of the biomolecules. At the solution concentrations studied here, QCM-D measurements also indicate that on both surfaces poly-L-lysine adsorbs to a greater extent than its constituent amino acid L-lysine. The opposite trend is demonstrated by poly-Lproline which sticks to both surfaces less extensively than its corresponding amino acid, L-proline. Both of these trends are explained by differences between the amino acids and their corresponding homopeptides in charge density, molecular mass, and solution concentrations. Additionally, we find that the adsorption of the molecules studied here can have a strong influence on interfacial water structure as detected in the SFG spectra.3