The adsorption of Rb+, Cs+, Mn2+, Co2+ and Yb3+ onto the positively charged hen egg-white lysozyme (HEWL) has been investigated by solving 13 X-ray structures of HEWL crystallized with their chlorides and by applying electrospray ionization mass spectrometry (ESI-MS) first to dissolved protein crystals and then to the protein in buffered salt solutions. The number of bound cations follows the order Cs+ < Mn2+ ’ Co2+ < Yb3+ at 293 K. HEWL binds less Rb+ (qtot = 0.7) than Cs+ (qtot = 3.9) at 100 K. Crystal flash-cooling drastically increases the binding of Cs+, but poorly affects that of Yb3+, suggesting different interactions. The addition of glycerol increases the number of bound Yb3+ cations, but only slightly increases that of Rb+. HEWL titrations with the same chlorides, followed by ESI-MS analysis, show that only about 10% of HEWL binds Cs+ and about 40% binds 1–2 Yb3+ cations, while the highest binding reaches 60–70% for protein binding 1–3 Mn2+ or Co2+ cations. The binding sites identified by X-ray crystallography show that the monovalent Rb+ and Cs+ preferentially bind to carbonyl groups, whereas the multivalent Mn2+, Co2+ and Yb3+ interact with carboxylic groups. This work elucidates the basis of the effect of the Hofmeister cation series on protein solubility.