New solid acid catalysts, consisting of heteropoly acid (HPA) H3PW12O40 (PW) supported on a mesoporous pure-silica molecular sieve MCM-41,have been prepared and characterized by nitrogen physisorption, X-ray diffraction, FT-IR, and P-31 magic angle spinning NMR. The PW/MCM-41 compositions with PW loadings from 10 to 50 wt% have similar to 30 Angstrom uniformly-sized mesopores. HPA retains the Keggin structure on the MCM-41 surface and forms finely dispersed HPA species. No HPA crystal phase is developed even at HPA loadings as high as 50 wt%. PW/MCM-41 exhibits higher catalytic activity than H2SO4 or bulk PW in liquid-phase alkylation of 4-t-butylphenol (TBP) by isobutene and styrene. In the alkylation of TBP by styrene, PW/MCM-41 shows a size selectivity compared to bulk PW and PW/SiO2, providing higher yields of a 2-(1-phenylethyl)-4-t-butylphenol, at the expense of the more bulky 2,6-bis-(1-phenylethyl)-4-t-butylphenol. The PW/MCM-41 compositions, having strong acid sites and a regular mesoporous system, are promising catalysts for the acid-type conversion and formation of organic compounds of large molecular size.