VSi beta zeolites prepared by a two-step postsynthesis method have been characterized by physical techniques. A significant reduction of intensity of the IR band near 3515 cm(-1) after impregnation of dealuminated P zeolite with aqueous NH4VO3 indicates that V ions specifically react with hydrogen-bonded SiO-H groups of vacant T sites. IR bands at 3618 and 3645 cm-1 are assigned to SiO-H groups interacting with V and to VO-H groups, respectively. In VSi beta, diffuse reflectance UV-visible data show that below 1.9 wt % V is present as lattice tetrahedral species and at higher content as extra-lattice octahedral species (mononuclear and polynuclear). VSi beta samples are EPR-silent at 298 or 77 K suggesting that there are no paramagnetic V-IV ions. IR studies show that V-OH groups are less acidic than Si-OH-Al groups of parent HA1 beta zeolite. IR results of CO adsorption evidence three kinds of Lewis acidic sites, related to lattice mononuclear and extra-lattice mononuclear and polynuclear V species. Quantitative IR studies of ammonia and pyridine adsorption reveal that only about half of V introduced into zeolite is able to form either Bronsted or Lewis acidic sites.