The autopsy samples of human aortic wall were investigated to determine the structure of the inorganic deposits formed at the onset of the mineralization process. The studies were concentrated on the material for which histochemical staining of randomly selected sections did not reveal the presence of minerals. The highly mineralized human media samples as well as broad spectrum of model compounds were investigated for comparative purposes. It was found that the inorganic phase at the onset of tunica media mineralization is composed of a few compounds including octacalcium phosphate as the predominant Ca-P compound, and a Mg-compound, probably magnesium carbonate or double/triple salts containing magnesium carbonate.