A study was conducted regarding the adsorption of peptides on commercially pure (cp) Ti and Ti-6Al-4V. The peptides used were arginine-glycine-aspartic acid-alanine (RGDA), arginine-glycine-aspartic acid-serine (RGDS), and arginine-phenylalanine-aspartic acid-serine (RFDS). The tripeptide RGD is known to be important for biologically specific adhesion reactions. This research was conducted to investigate the reason for a tendency toward thrombus formation with Ti-6Al-4V that is not observed with cp Ti. After argon plasma cleaning, coupons of the titanium alloys were inserted into solutions with variable concentrations (0.0625-2 mg/ml) of an individual peptide group under constant temperature and time conditions. The samples were rinsed, dried, and analyzed with x-ray photoelectron spectroscopy (XPS). Adsorption isotherms were obtained by plotting the relative amount of peptide adhesion as a function of solution concentration. It was postulated through the XPS and adsorption isotherm data that the major adhesion mechanism for the peptides to the titanium alloys was hydrogen bonding. CP titanium and Ti-6Al-4V are hypothesized to react differently as implants because Ti-6Al-4V has a more electropositive surface, which allows fewer hydrogen bonds to form. Hydrophilic reactions were proposed to be of secondary importance during bioadhesion, influencing the structure of the second layer adsorbed. There was no correlation found between the net charge of the peptide groups and their adhesion to the alloys.