Tungsten oxide gels and films were prepared by the sol-gel process using 2,4-pentanedione (PTN) as an organic ligand. WO3 powders were obtained by peeling the films from the quartz glass substrates. The crystal structure of both the gels prepared with and without PTN and fired at 700 degrees C was monoclinic. WO3 powders peeled from film samples prepared both with and without PTN showed only the monoclinic crystal structure, whereas the crystal phases of both types of films were cubic and a mixture of cubic and monoclinic crystals, respectively. These results indicate that the cubic crystals of the WO3 in the films were transformed to the monoclinic crystals of the powders after the films were peeled from the quartz glass substrates. It is concluded that the cubic crystals in the films are transformed to stable monoclinic crystals by the peeling process, whereas PTN and the quartz glass substrate can control the crystal phase of WO3 films selectively to form the cubic structure.