The dehydration of molybdic acid, MoO3-H2O, was studied by thermal analysis, X-ray diffractometry and FTIR spectroscopy. The results show that an intermediate phase, MoO3-2/3H(2)O is formed at 216 degrees C and the monoclinic form of MoO3 is grown above 350 degrees C. The mechanism of dehydration and structural rearrangement were confirmed by the features of the infrared spectra showing formation of corner-shared MoO6 octahedra.