Ultrathin films (5 nm, 10 nm and 20 nm effective thickness) of WO3 have been deposited in high vacuum (10(-6) Torr) onto single crystal Si(100) substrates and studied with X-ray diffraction, atomic force microscopy, scanning tunneling microscopy and spectroscopy. The experiments have been carried out on "as-deposited" thin films or after I h post-deposition annealing at various temperatures (ranging from 300 degrees C to 500 degrees C). A size induced increase of the amorphous to crystalline (monoclinic) phase transition has been observed for the 5 nm and 10 nm films, with a critical crystallite size of 25 +/- 5 nm and a critical temperature of 345 +/- 5 degrees C. All the experimental evidences show that, upon annealing, there is a diffusion limited aggregation growth of WO3 that forms large flat two-dimensional islands composed by aggregates of individual crystallites approximately uniform in size and shape. These islands are isolated in the 5 nm thin films, are connected in the 10 mu case and form a uniform patchwork in the 20 mu thin films. Scanning tunneling spectroscopy shows the opening of a large surface band gap (2.7 eV) in the 500 degrees C annealed films and the significant presence of in gap states for thin films prepared with a lower (below 400 degrees C) annealing temperature. These findings are discussed in view of the optimization of the best morphological, structural and electronic parameters to fabricate WO3 gas sensing devices at the sub-micrometer length scale. (c) 2005 Elsevier B.V. All rights reserved.