From solutions of ammonium molybdate this salt was precipitated with acetone. Decomposition in air at 330 °C yields MoO3 with a specific surface area of 17 m2/g, by means of reduction with hydrogen 4.5, 4 and 3.5 valent oxides may be formed with 30, 50 and 80 m2/g, respectively. Molybdenum (VI) oxide present as a monomolecular layer on supports is much less readily reduced than unsupported MoO3. This difference reflects the interaction of Mo ions with ions of the support. The reducibilities of the various catalysts show that Mo(VI) oxides interact far less with SiO2 than with Al2O3, CeO2 and ZrO2. This is also reflected in the rates of 2-propanol decomposition on these catalysts, the activities on MoOx and on MoOx−SiO2 being much higher than on the other catalysts.