Tungstated zirconia catalysts with loadings ranging from near monolayer (6 wt% W) to multilayer coverages (30 wt% W) were prepared by wet impregnation with ammonium metatungstate as precursor, characterized by X-ray diffraction, temperature-programmed reduction, and in situ X-ray photoelectron spectroscopy and compared for surface reducibility. Two catalyst preparation methods were used. On the one hand, the ammonium metatungstate precursor was deposited directly on commercial monoclinic ZrO2, or it was deposited on amorphous ZrOx(OH)(4-2x) hydroxide, according to the Hino and Arata procedure and leading to tetragonal ZrO2. The degree of reducibility of the WO3/ZrO2 systems and consequently the catalytic behavior (forthcoming Part II) strongly depend on the initial WO3 content but not at all on the monoclinic or tetragonal structure of ZrO2. Whatever the preparation method, the surface of the near-saturation monolayer catalyst is mainly constituted of amorphous tungstate species in which tungsten atoms (W6+ and W5+) are in tetrahedral coordination, in direct interaction with the support and thus difficult to reduce. The increase in tungsten content yields a progressive formation of tungstate species in which tungsten is octahedrally coordinated, more easy to reduce, and giving W4+ surface species. Increasing the WO3 content above 15 wt% W leads to the formation of bulk-like WO3 crystallites, whose reduction leads to the appearance of metallic tungsten. (C) 2004 Published by Elsevier Inc.