The potassium and proton mixed salt of mono-Nb substituted Keggin-type phosphomolybdate, KH3[PMo11NbO40], was isolated in a pure form by reacting Keggin-type phosphomolybdic acid (H3[PMo12O40]) and potassium hexaniobate (K8Nb6O19) in water, followed by freeze-drying. The all protonic form, H4[PMo11NbO40], was isolated via proton exchange with H-resin and subsequent freeze-drying. The most crucial factor to isolate KH3[PMo11NbO40] and H4[PMo11NbO40] in pure forms is the evaporation of water using the freeze-drying method. Using a similar procedure, the potassium salt of the di-Nb substituted compound K5[PMo10Nb2O40] was isolated. H4[PMo11NbO40] exhibited high catalytic activity for oxidizing isobutylaldehyde to methacrolein and moderate catalytic activity for the Wacker-type oxidation of allyl phenyl ether when combined with Pd(OAc)2.