Molybdenum trioxide (MoO3) was evaporated from a tungsten resistive source in presence of hydrogen as carrier gas at pressures from 100 Pa to 600 Pa. Crystalline samples of molybdenum oxide were obtained at source temperatures below 950 degrees C. However, at source temperatures above 1000 degrees C, nanoporous structures were obtained, with porosity in the range of 10-85 nm. As shown by X-ray diffraction as well as selected area electron diffraction (SAED), the nanoporous material was amorphous. Under transmission electron microscopy (TEM) examination, and due to the electron beam, progressive formation of crystalline clusters was observed, as revealed by sequential changes of SAED and dark field images and corroborated in the high resolution TEM images (HRTEM). The growth of crystalline clusters at the nanometer scale occurs without destruction of the nanoporous structures.