The effect of sodium addition on the physicochemical properties of Mo/SnO2 catalysts was studied. The catalysts were prepared by precipitation using aqueous solution of ammonium heptamolybdate in two Mo amounts (5.5 and 14.5% (wt)); afterwards they were impregnated with sodium ion (1 % wt). The solids were characterized by nitrogen adsorption, X-ray diffraction, UV-vis diffuse reflectance spectroscopy, FTIR, and FTIR of adsorbed pyridine in order to evaluate changes on Mo species and textural and acidic properties. The results have shown that Mo/SnO2 samples present dispersed isolated tetrahedral Mo species. The addition of sodium ion to Mo/SnO2 catalysts promotes the transformation of dispersed isolated tetrahedral Mo species into dispersed two-dimensional polymolybdates and also causes changes in textural and acidic properties of these Mo/SnO2 catalysts. (c) 2006 Elsevier B.V. All rights reserved.