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The formation, composition and shapes of small particles of tungsten oxide, which are formed by the evaporation of tungsten trioxide (WO3) powder onto silicon monoxide substrate film at various air pressures, are investigated with a transmission electron microscope. WO3 crystals of about 0.15 to 0.015 μm in diameter are formed on the substrate film at air pressures of 101.3 to 1.3 kPa, and the size of the crystals becomes smaller with decreasing air pressure. If the air pressure is lower than 1.3 kPa, the evaporated tungsten oxide is liquid in the gas phase and condenses as amorphous particles of 50 A diameter on the substrate films. At an air pressure of 1.3 to 0.13 Pa, tungsten oxide drops are formed on the substrate monoxide film and become hemispherical or flat truncated-sphere-shaped particles when the diameters are smaller or larger than about 700 Å respectively. The drops condensed on the substrate are heated in various vacua and it is noted that the drops contain much smaller amounts of oxygen than do WO2 crystals.