Tungsten oxide powders with controlled micro-to-nanoscale features, i.e. from microspheres to mesoporous structure, have been synthesized using peroxopolytungstic acid (PTA) and thiourea. Using PTA alone, elliptical tungsten oxide microspindles are hierarchically assembled from rectangular nanoparticles (NPs) after heat treatment at 450 degrees C. With an appropriate addition of thiourea into the PTA precursor, hierarchical microspheres are obtained through the aggregation of isotropic NP unit blocks. When the thiourea content in the precursor increases further, high-surface-area mesoporous tungsten oxides composed of well-dispersed isotropic NPs can be synthesized. We suggest that thiourea acts as a capping agent for forming the isotropic NP unit blocks in the precursors. Moreover, gaseous species released at high temperatures through decomposition and oxidation of nonchelated thiourea in the precursor will suppress the aggregation of NP unit blocks. As a result, tungsten oxide powders in form of microspheres or mesoporous nanostructures can be tailored by the thiourea content in the precursor.