The effect of Mg in Ag(Mg)/SiO2/Si multilayers on the adhesion, passivation, and resistivity following vacuum annealing at 200–500 °C has been investigated. The annealing of Ag(Mg)/SiO2/Si multilayers produced surface and interfacial MgO layers, resulting in a MgO/Ag/MgO/SiO2/Si structure. The formation of a surface MgO/Ag bilayer structure provided excellent passivation against air and CF4 plasma chemistry. In addition, the adhesion of Ag to SiO2 was improved due to the formation of an interfacial MgO layer resulting from the reaction of segregated Mg with SiO2. However, the negligible solubility of Si in Ag prevented the dissolution of free silicon into the Ag(Mg) film produced from the reaction Mg + SiO2 = MgO + free Si, which in turn limited the reaction between Mg and SiO2, which led to a decrease in the adhesion of Ag to SiO2 at the higher temperature. The use of an O2 plasma prior to Ag(Mg) alloy deposition on SiO2 produced an oxygen-rich surface on the SiO2, which allowed for the enhanced reaction of the segregated Mg and SiO2 at the surface, thus resulting in markedly increased adhesion properties.