The reversible intercalation of multivalent cations, especially Mg2+, into a solid-state electrode is an attractive mechanism for next-generation energy storage devices. These reactions typically exhibit poor kinetics due to a high activation energy for interfacial charge-transfer and slow solid-state diffusion. Interlayer water in V2O5 and MnO2 has been shown to improve Mg2+ intercalation kinetics in nonaqueous electrolytes. Here, the effect of structural water on Mg2+ intercalation in nonaqueous electrolytes is examined in crystalline WO3 and the related hydrated and layered WO3.nH(2)O (n = 1, 2). Using thin film electrodes, cyclic voltammetry, Raman spectroscopy, X-ray diffraction, and electron microscopy, the energy storage in these materials is determined to involve reversible Mg' intercalation. It is found that the anhydrous WO3 can intercalate up to 0.3 Mg2+ (75 mAh g(-1)) and can maintain the monoclinic structure for at least SO cycles at a cyclic voltammetry sweep rate of 0.1 mV s-1. The kinetics of Mg2+ storage in WO3 are limited by solid-state diffusion, which is similar to its behavior in a Li+ electrolyte. On the other hand, the maximum capacity for Mg' storage in WO3.nH(2)O is approximately half that of WO3 (35 mAh g(-1)). However, the kinetics of both Mg2+ and Li+ storage in WO3.nH(2)O are primarily limited by the interface and are thus pseudocapacitive. The stability of the structural water in WO3.nH(2)O varies: the interlayer water of WO3.2H(2)O is removed upon exposure to a nonaqueous electrolyte, while the water directly coordinated to W is stable during electrochemical cycling. These results demonstrate that tungsten oxides are potential candidates for Mg2+ cathodes, that in these materials structural water can lead to improved Mg2+ kinetics at the expense of capacity, and that the type of structural water affects stability.