In order to develop a new kind of dissolvable alloy used for hydraulic fracturing tools, a series of Al-Mg-Ga-In-Sn alloys with different Mg contents were prepared by smelting and casting methods to investigate the effect of Mg on the reactivity of alloys with water. Their microstructures were investigated by means of XRD and SEM. The Volta potential difference (Delta VPD) of the interfacial phases with Al was measured using AFM/SKPFM. Mg forms different intermetallic compounds with low melting point metals, depending on Mg additions in alloys. Different from the formation of In3Sn phase in Al-Ga-In-Sn alloys, the In3Sn phase disappears in Mg-bearing alloys. The experiments found that the reactivities of Mg-bearing alloys with water are closely related to Mg content of alloys and water temperature. The alloys react with water slowly at the beginning of reaction, following with a fast reaction. From the corrosive morphologies of alloys, the precipitation of low melting point metals from interfacial phases was observed at Al grain boundaries. The lower electric potential of interfacial phases than Al was accounted for the precipitation of low melting point metals, because the interfacial phases form micro-galvanic cells with Al when alloys contact with water. A reaction mechanism of Mg-bearing alloys with water was proposed to explain the reactivties of alloys with water. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.