Gallium-67 is widely used as an imaging agent for tumors and inflammatory abscesses. It is well established that Ga3+ travels through the circulatory system bound to the serum iron transport protein transferrin and that this protein binding is an essential step in tumor localization. However, there have been conflicting reports on the magnitude of the gallium-transferrin binding constants. Therefore, thermodynamic binding constants for gallium complexation at the two specific metal binding sites of human serum transferrin at pH 7.4 and 5 mM NaHCO3 have been determined by UV difference spectroscopy. The conditional constants calculated for 27 mM NaHCO3 are log K1 = 20.3 and log K2 = 19.3. These results are discussed in relation to the thermodynamics of transferrin binding of Fe3+ and to previous reports on gallium binding. The strength of transferrin complexation is also compared to that of a series of low molecular weight ligands by using calculated pM values (pM = -log [Ga-(H2O)6]) to express the effective binding strength at pH 7.4.