Three bone graft substitute materials (Bioglass 45S5, Bio-Oss® and Algipore®) were conditioned in α-minimum essential medium (α-MEM), with the addition of 10% fetal bovine serum (FBS), for 1 and 7 days. The chemistry of their solid-solution interface was monitored by X-ray photoelectron spectroscopy, using fast-frozen sample technique, and compared to that reported for original α-MEM. FBS added to the biological medium causes significant changes in the interface after only 1 day of conditioning. Interfacial chemical composition and N 1s spectra show immediate adsorption of proteins at the surface of all three biomaterials, independent of their surface charge and chemical composition. However, the atomic ratio C/N and the C 1s spectra indicate a different orientation of adsorbed serum proteins, which is dependent on the particle's surface charge. Moreover, the adsorption of serum proteins at the surface of Bio-Oss causes a charge reversal at the interface, as evidenced by the change in the atomic ratio of Na/Cl. In addition to the particle's surface charge, the formation of the protein interfacial layer at the surface of the biomaterial seems to be the second major phenomenon important for subsequent cell recognition and the initiation of biomineralization. Copyright © 2012 John Wiley & Sons, Ltd.