The microstructures of commercially important bismuth molybdate catalysts in relation to olefin oxidation reactions are examined by electron microscopy (EM) techniques. The microstructural characterization has been carried out using dynamic ( in situ) EM, high resolution EM, and microanalysis. The coprecipitated catalyst system Bi 2MoO 6 or γ, together with the γ phase, contains small amounts of tetragonal Bi 2MoO 6 phase, Bi 2Mo 3O 12 (α phase), Bi 2O 3, and MoO 3. In reduction with propylene, at catalyst operating temperatures of 400-500°C, in the dynamic experiments conducted on α- and γ-phase crystallites under reaction conditions no evidence for extended defects such as crystallographic shear planes has been obtained, instead an ordered intermediate phase similar to (101) Bi 2Mo 2O 9 (β phase) is observed which is found to be unstable. Observations by electron microscopy have been confirmed with parallel measurements made in a reactor connected to a gas chromatograph and mass spectrometer system. The possible influence of the microstructural changes on the catalytic behavior of the system is examined.