Friedel-Crafts-type reaction of p-xylene with ␥-butyrolactone or vinylacetic acid over H 4 SiW 12 O 40 supported on SiO 2 have been investigated with different loadings of H 4 SiW 12 O 40 . The catalytic performance of H 4 SiW 12 O 40 /SiO 2 have been compared with those of typical solid acid catalysts such as zeolites, ion-exchanged resins, and binary oxides. H 4 SiW 12 O 40 supported on SiO 2 was superior in activity and selectivity to 5,8-dimethyl-␣-tetralone compared to other catalysts for the reaction between p-xylene and ␥-butyrolactone. SiO 2 -supported H 4 SiW 12 O 40 also was efficient for the formation of 3,4,7-tetramethyl-␣-indanone from p-xylene and vinylacetic acid. The formation of 5,8-dimethyl-␣-tetralone is extremely sensitive to the load of H 4 SiW 12 O 40 on SiO 2 , reaching a maximum at approximately 30 wt%, due to the change in the acid amount of the outermost surface as determined by benzonitrile-temperature programmed desorption. By analogy with reaction between 1,3,5-trimethylbenzene and ␥-butyrolactone, the reaction pathway involves initial alkylation of p-xylene with ␥-butyrolactone, followed by intramolecular acylation to 5,8-dimethyl-␣-tetralone, which was supported by quantum calculations of ␥-butyrolactone and protonated ␥-butyrolactone.