The adsorption of proteins is the first process to take place when a solid is immersed in a biological fluid; though not yet thoroughly understood at a molecular level, this process is also known to be strongly influenced by the presence of salt in solution or by pH changes. In the present work, poly-L-glutamic acid (PG) and poly-L-lysine (PL) were selected to mimic the behaviour of some protein fragments. Their adsorption was investigated by infrared spectroscopy in various modes, both on planar and on divided (powder) surfaces of aluminium oxide. These two peptides were shown to have different behaviours when adsorbed from solutions containing CaCl2 or not, and at various pH values. Polarisation Modulation-Reflection Absorption InfraRed Spectroscopy (PM-RAIRS), applied in a special cell designed to characterize the solid surface in contact with the liquid, enabled the observation of the influence of pH and salts upon polypeptide adsorption. At pH values higher than 5, and in the presence of CaCl2 in solution, a net increase of the PG adsorbed amount is observed, whereas no such effect could be detected for PL. Specific interactions between the COO- groups on the side chains and the surface, or between those of two different molecules, was inferred. Interestingly, similar conclusions could be drawn for the surface of alumina powders contacted with solutions of PG and PL and characterised by Attenuated Total Reflectance (ATR)- IR. This work demonstrates the potential for IR investigations of solid oxide-liquid interfaces combining the study of planar and finely divided surfaces.