The formation of zinc hydroxide nitrate, Zn5(OH)8(NO3)2·2H2O, by reaction between zinc oxide and aqueous zinc nitrate solution was examined. Scanning electron microscopy, X-ray diffraction and thermogravimetric analysis were used to analyze the conversion of nanoscale ZnO particles into much larger crystals of the hydroxide nitrate. The rate of the reaction displayed sigmoidal behavior with the maximum conversion rate at ca. 75 min. The reaction stoichiometry involves a 1:1 ZnO/Zn(NO3)2 molar ratio. The data indicate that an amorphous zinc-containing intermediate phase is formed during the transition, and that the zinc hydroxide nitrate crystals nucleate and grow from this phase. The crystals of zinc hydroxide nitrate are several µm in size, but are formed from zinc oxide crystals of only a few hundred nanometers in size, indicating that mass transfer in the aqueous phase plays an important role. We propose that H+-catalyzed dissolution/precipitation is the key process in the mechanism of the reaction. The zinc hydroxide nitrate is stable to about 110 °C, but decomposes above that temperature to a series of less hydrated phases, with associated loss of mass, until zinc oxide is formed at about 190 °C. The solubility product, Ksp, of Zn5(OH)8(NO3)2·2H2O in water was measured by two independent techniques and found to be in the range of 7.4–8.5 × 10–11.