The effect of the presence of NaCl, CaCl2, or MgCl2 at the same ionic strength on the structure of beta -casein layers adsorbed on hydrophobic surfaces has been investigated by neutron reflectivity measurements. The data were fitted to a four-layer model. The volume fraction versus distance profiles have a similar shape whether beta -casein is adsorbed from NaCl, CaCl2, and MgCl2 of the same ionic strength or whether the protein concentration is lowered 10 times. In particular at larger distances from the surface, the volume fraction values are low and similar. However, close to the hydrophobic surface the volume fraction of protein decreases in the order CaCl2 > MeCl2 > NaCl. We have also used a specific proteolytic enzyme, endoproteinase Asp-N, which cleaves off the hydrophilic part of beta -casein, as a tool to reveal the interfacial structure of the protein. For all the different types of added electrolytes, endoproteinase Asp N only affects the outermost beta -casein layer. Subsequent addition of beta -casein in all cases led to large increases in amounts adsorbed and in the thickness of the outer layers.