Hydrogen-bond-assisted molecular aggregation of decavanadate anions, [HnV10O28]((6-n)-), in nonaqueous solutions was probed by systematic small-angle X-ray scattering and H-1 and V-51 NMR spectroscopic measurements in mixtures of acetone and 1,4-dioxane. Under acetone-rich conditions, the decavanadate anion prefers a self-associated hydrogen-bonded dimer of {[H3V10O28](2)}(6-), which dissociates into monomeric species as the proportion of 1,4-dioxane increases. The association/dissociation behaviors of the decavanadate anions were proven to be reversible and driven by the protophobic/protophilic nature of the solvent.