Different sizes of single-crystal tungsten trioxide nanocuboids with monoclinic structure have been successfully synthesized via one-step hydrothermal procedure with and without the addition of cationic surfactant cetyltrimethylammonium bromide (CTAB) at 453 K. The as-synthesized products were characterized by X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy (TEM), high-resolution TEM, and UV-visible absorption spectra. The results showed that the size of the WO3 nanocuboids can be simply controlled by varying the molar ratio of CTAB to tungstenic acid. A solid-solution-solid mechanism of crystal growth was discussed. The as-prepared WO3 nanocuboids were further applied in gas sensors, displaying promising selectivity and high sensitivity to various organic gases such as petrol and ethanol. (c) 2006 The Electrochemical Society.