Monodispersed fine particles of (NH4)(3)PW12O40 having regular shape were successfully prepared as precipitates by the titration of NH4HCO3 solution to H3PW12O40 solution. The particles were ca. 1 mu m in size and microporous with 151 m(2) g(-1) of BET surface area. AFM image demonstrated that these particles were aggregates of nanocrystallites of ca. 15 nm in size.