This paper is the first part of a systematic study of the influence of W and Ir loading on the activity of Ir/WO3/ZrO2-SiO2 catalysts for the ringopening reaction of naphthenic molecules using methylcyclohexane (MCH) as a model compound. A series of Si-stabilized tungstated zirconias, WOx/ZrO2-SiO2, containing up to 3.5 at W/nm(2) was prepared. Ir-based catalysts containing up to 1.2 wt% were obtained by impregnation of these solids. The acidity of Si-stabilized tungstated zirconias, ZSiWx, was monitored by low-temperature CO adsorption followed by infrared (IR) spectroscopy. The results indicated the development of strong and relatively strong Bronsted acidity with increasing W surface density above a threshold of I atom W/nm2. The study of the Ir-doped solids showed that the texture, structure, and acidity of the ZSiWx solids were not affected by Ir deposition. The characterization of the metallic function was performed by hydrogenation of toluene and by CO adsorption at ambient temperature, followed by IR spectroscopy. Both methods indicated a low and constant Ir dispersion for the yIr/ZSiWO (y = 0.3, 0.6, 1.2) series. For a given Ir loading (1.21r/ZSiWx), a progressive decrease in the dispersion of Ir metal with increasing W surface density was observed. (c) 2006 Elsevier Inc. All rights reserved.