The process of reduction of tungsten trioxide in both dry and wet hydrogen was studied by means of the weight loss measurement, the X-ray analysis, the surface area measurement and the observation by electronmicroscope. The results obtained are as follows.
(1) The reduction rate differs from each stage of the reaction mentioned above and it also changes with the time duration of reduction. However, the reduction rate of α-tungsten oxide to metallic tungsten maintains a constant value until the reaction almost completes.
(2) The average size of the tungsten particles produced by the reduction in hydrogen is affected mainly by the reduction temperature and also by the partial pressure of water vapor in hydrogen.