X-ray photoelectron spectroscopy (XPS) measurements on SiO2-ZrO2 mixed oxides with varying Si/Zr ratios are presented. The measurements yield interesting insight into surface properties which may help the understanding of their catalytic action. Especially, the surface acid properties of the catalysts are better understood. On the one hand, on SiO2-ZrO2 mixed oxides with 75 wt% or less ZrO2, an increase of the positive charge on the Zr cation is observed compared to the single oxide ZrO2. This is concluded from higher Zr3d(5/2) binding energies. The higher binding energy (charge) is indicative of strong Lewis acidity in the mixed oxides. On the other hand, oxygen associated with the Si cation in these mixed oxides appears to have a slightly higher electron density (base strength) than in the single-oxide SiO2, as is concluded from the lower O1s binding energies (of oxygen near Si). This fact is an indicator for a small increase in the number of Bronsted acid sites. Furthermore, XPS shows that the surface of the mixed oxides containing 75 wt% or less ZrO2 is depleted in zirconium. The surface Zr concentration determined with XPS is roughly 50% of the bulk value. This depletion is most probably due to differences in reactivity of the Si and the Zr precursors in the precipitation step and not due to phase separation in the calcination step. A noncalcined catalyst also shows surface depletion in Zr.