Two types of molybdenum trioxide were synthesized by evaporating the molybdic acid solution prepared via cation-exchange of an aqueous solution of Na2MoO4 center dot 2H(2)O. When the molybdic acid solution was evaporated to dryness at 323 K under reduced pressure and then calcined at 573 K in a stream of oxygen, alpha-MoO3 was exclusively produced. However, addition of HNO3 to the solution before the evaporation resulted in formation of a bright yellow powder which was identified as beta-MoO3. The phase ratio of beta/alpha was dependent on the quantity of HNO3; pure beta-MoO3 was successfully synthesized when the molar ratio of HNO3/Mo in the solution was in the range of 1-2. It was also found that the HNO3 addition made no structural change of the isopolymolybdates in the solution and the dried precursor, but did affect the dehydration of the dried precursor to induce the crystallization of the beta-phase. In partial oxidation of methanol, beta-MoO3 exhibited much higher methanol conversion than the alpha-form. IR spectra of chemisorbed pyridine elucidated that the higher catalytic activity was caused by the relatively large number and high acidity of Lewis acid sites on the surface of beta-MoO3 (c) 2007 Elsevier B.V. All rights reserved.