The surface structure and morphology of WO3(1 0 0) thin films were studied using scanning tunneling microscopy (STM) and low-energy electron diffraction. The films experienced a net-reducing environment when annealed in oxygen at 800 K leading to surface phase transitions from p(2 X 2) to p(4 X 2), and from p(4 X 2) to a mix p(4 X 2) and p(3 X 2). Increasing the annealing temperature to 830 K in ultra-high-vacuum (UHV) led to a fully p(3 X 2) reconstructed surface. Continued UHV annealing above 800 K caused (I X 1) islands to appear on the p(3 X 2) surface and the film color to darken. Eventually, prolonged UHV annealing led to a (I X I)-terminated surface with straight steps oriented in [0 0 1] or [0 1 0] directions due to crystallographic shear planes. The randomly spaced steps on the (I X 1) surface indicated variations in the local stoichiometry in the film. An added row model proposed for the p(4 X 2) structure is also shown to be consistent with the p(3 X 2) structure. The formation of the P(4 X 2) structure from the p(2 X 2) structure was attributed to W5+ migration into the bulk to form the troughs between the added rows. Reduction of the p(4 X 2) structure caused the troughs to narrow rather than deepen, suggesting that W5+ or added row surface diffusion competes with migration of reduced W ions into the bulk when the p(3 X 2) structure forms. The STM images also show evidence that the (I X 1) structure forms through coalescence of the added rows. (C) 2003 Elsevier B.V. All rights reserved.