Nanocrystalline ZrO2 and WOx–ZrO2 catalysts with good control over the zirconia phase and the nature of the W species were synthesized by a microwave-assisted aqueous route. The monoclinic ZrO2 (m-ZrO2) and tetragonal ZrO2 (t-ZrOz) polymorphs were isolated through accurate control of the synthetic conditions, in acidic and basic media, respectively. The evolution of the zirconia and W species under annealing of the WOx–ZrO2 nanocomposites at 500 °C and 800 °C was studied, and the resulting materials were tested as catalysts for aldoxime dehydration and hydration of alkynes for the production of nitriles and carbonyls, respectively. In the dehydration reaction, the most active species are W(VI) in tetrahedral coordination, independently of the ZrO2 polymorph, while in the hydration reaction the zirconia phase plays a key role, as the tungsten doped t-ZrO2 appears to be the most active catalyst.