In this study, MoO3/ZrO2 catalysts have been prepared by co-precipitation at different Mo/Zr ratios and at different digestion times of the catalyst precursors. MoO3/ZrO2 catalysts have higher surface areas than pure ZrO2. As the MoOs content increases, the surface area of the samples increases and reaches a maximum. The value of this maximum and the correspondent MoO3 content depend on the calcination temperature. Addition of Mo also preserves ZrO2 in the metastable tetragonal phase instead of turning into the monoclinic phase. A minor modification of the preparation procedure has a tremendous effect on the surface areas of the catalysts. Samples obtained by addition of (NH4)(6)Mo7O24 after formation of the Zr(OH)(4) gel have larger surface areas than those obtained by co-precipitation. The surface areas of MoO3/ZrO2 decrease when the digestion time of the catalyst precursors is increased from 0 to 72 h. For long digestion times, the XRD patterns clearly show the formation of Zr(MoO4)(2) at high MoOs contents. In addition, Mo cannot oppose the tetragonal to monoclinic transition. Mo-Zr interactions seem to be the key factors in the different results obtained here, as suggested by XRD and TPR.