A very smooth amorphous molybdenum oxide thin film can be fabricated easily by a spin-coating technique using a solution of peroxo-polymolybdic acid, which i formed by the direct reaction of metallic molybdenum with hydrogen peroxide. Amorphous MoO3 . nH2O resulting from heating such a film at 80-150-degrees-C exhibited reversible electrochromism in an organic electrolyte solution. However, its electrochromic characteristics depended on the heating temperature. Coloration dynamics of films treated at various temperatures were investigated by a potential step method. When the film was treated at temperatures as high as 150-degrees-C, its coloration process was diffusion controlled, while the transient current observed with films treated at lower temperature did not obey the Cottrell-type relationship. Such behavior is discussed in relation to the structural features of the amorphous MoO3 . nH2O films.