AbstractAlthough certified magnesium-based implants are launched some years ago, the not well-defined Mg degradation mechanism under physiological conditions makes it difficult to standardize its use as a degradable biomaterial for a wide range of implant applications. Among other variables influencing the Mg degradation mechanism, monitoring the pH in the corrosive solution and, especially, at the corroding interface is important due to its direct relation with the formation and stability of the degradation products layer. The interface pH (pH at the Mg/solution interface) developed on Mg-2Ag and E11 alloys are studied in situ during immersion under dynamic conditions (1.5 mL min-1) in HBSS with and without the physiological amount of Ca2+ cations (2.5 × 10-3 m). The results show that the precipitation/dissolution of amorphous phosphate-containing phases, that can be associated with apatitic calcium-phosphates Ca10-x(PO4)6-x(HPO4 or CO3)x(OH or ½ CO3)2-x with 0 ≤ x ≤ 2 (Ap-CaP), promoted in the presence of Ca2+ generates an effective local pH buffering system at the surface. Thus, high alkalinization is prevented, and the interface pH is stabilized in the range of 7.6 to 8.5.