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Magnesium oxide (MgO) and hydroxide [Mg(OH) 2 ] are conventionally considered insoluble in water and stable at high temperatures. However, in this study, we found significant dissociation of MgO and Mg(OH) 2 into ions when they were immersed in different physiologically relevant solutions in the form of 20nm and 10-nm nanoparticles respectively, under standard cell culture conditions in vitro, i.e., a 37°C, 5% CO 2 /95% air, sterile, humidified environment. The change in Mg 2+ ion concentrations and pH measured in the physiologically relevant solutions (e.g., Dulbecco's modified Eagle's Medium (DMEM), simulated body fluid (SBF), relevant chloride solutions, and deionized water) confirmed their dissociation. Possible mechanisms and contributing factors for dissociation of MgO and Mg(OH) 2 nanoparticles were discussed. The evidence suggests that nucleophilic substitution of OH − by Cl − in Mg(OH) 2 is energetically unfavorable and it is more likely that Cl − plays a role in the stabilization of intermediate forms of MgO and Mg(OH) 2 as it dissociates. The pH and buffering capability of the immersion solutions might have played the most significant role in dissociation of these nanoparticles when compared with the roles of chloride (Cl − ), proteins, and different buffering agents. This article provided the first evidence on the dissociation of MgO and Mg(OH) 2 nanoparticles in physiologically relevant conditions and elucidated possible factors contributing to the observed behaviors of these nanoparticles in vitro, which is important for their potential medical applications in vivo. Keywords Magnesium oxide (MgO) nanoparticles . Magnesium hydroxide [Mg(OH) 2 ] nanoparticles . Biofluids . Dulbecco's modified Eagle's Medium (DMEM) . Simulated body fluid (SBF) . HEPES buffer . Chloride (Cl − ) solutions