Protein-bound calcium (prCa) constitutes about 40% of serum calcium, in which albumin is the most dominant protein. Given the chemical interaction between calcium and phosphate (Pi), the increased serum Pi in chronic kidney disease may cause changes in the composition and structure of the prCa fraction. Here, we report the Pi binding on the protein-bound calcium in uremic rat serum. Using adenine-fed rats as a uremic model, we determined the levels of calcium and phosphate fractions in rat serum by ultrafiltration, and found that the protein-bound phosphate (prPi) in the uremic serum was markedly higher than that in control rat serum. The elevated prPi level was comparable to the prCa level, indicating the formation of protein-bound calcium phosphate pr(Ca)j-m(CaPi)m. We then characterized pr(Ca)j-m(CaPi)m by ex vivo X-ray absorption near-edge structure spectroscopy, revealing the discrete state of the calcium phosphate clusters associated with protein. Finally, in a quantitative investigation using Ca- and Pi-boosted serum, we discovered the threshold concentration for the Pi binding on protein-bound calcium, and derived an equation that correlates pr(Ca)j-m(CaPi)m with the Ca´Pi product. The threshold, while preventing phosphate from binding to protein-bound calcium in normal condition, allows excess phosphate to do so. Thus, the formation of protein-bound calcium phosphate represents a way in which serum proteins mediate the levels of calcium and phosphate. It deserves further investigation whether the molar ratio of (prPi/prCa)×100% may serve as a serum index of the vascular calcification status in chronic kidney disease.