Abstract: Acidic cesium salts of dodecatungstophosphoric acid (H 3 PW) supported on silica and MCM-41 were characterized by multinuclear MAS NMR, FTIR, and catalytic test reactions. 31 P-NMR data indicate that the supported samples contain salts of various stoichiometries (Cs 3 PW and Cs 2 HPW) and even H 3 PO 4 . According to 29 Si-NMR data, deposition of the Cs salt does not change the relative amount of various SiOH groups in the case of the silica support. In contrast, when the support is MCM-41, the relative amount of the SiOH groups increases significantly, indicating the breaking of the siloxane bonds resulting from their interaction with H 3 PW. The 133 Cs MAS-NMR spectra also suggest that the CsPW salt interacts more strongly with the MCM-41 support than with SiO 2 . The results of catalytic studies show that cesium salts of dodecatungstophosphoric acid with low Cs content supported on silica or MCM-41 (Cs 1.7 H 1.3 [PW 12 O 40 ]-on-SiO 2 and Cs 1.8 H 1.2 [PW 12 O 40 ]-on-MCM41) are active, selective, and recyclable catalysts in Friedel–Crafts alkylations (isopropylation, transalkylation), the aromatic ortho -Claisen rearrangement, and the dehydration of pinacol. The specific activity of these catalyst preparations are comparable with that of neat Cs 2.6 H 0.4 [PW 12 O 40 ]. This indicates that the active species are in a well-dispersed form.