Aim. Calcium phosphate deposition in blood vessels is correlated to increased mortality risk. In this study, the formation of solid calcium phosphate in an in vitro solution mimicking the inorganic part of blood serum was studied. Methods. The precipitates formed were analyzed using several experimental techniques, including infrared spectroscopy, Raman spectroscopy, X-ray energy dispersive spectroscopy, chemical analysis of combustion gases, thermogravimetric analysis, as well as transmission electron microscopy and scanning electron microscopy. Results. The results indicate a microscopically amorphous but nano-crystalline material with an overall apatite structure. A plausible stoichiometry was determined to Ca5(PO4)3(HCO3).4H2O with an estimated solubility constant of 6.10-39 (mol/L)9. Bicarbonate in the water solution was shown to be essential for the precipitation, giving implications for in vitro studies. Conclusions. The calcium phosphate formed in this study shows many similarities to pathological calcium phosphates regarding composition, morphology and crystallinity.