Zeolite loaded with alkylsilane-covered titanium oxide was found to be more effective than nonporous silica counterpart as “phase-boundary catalyst (PBC)” to promote the epoxidation of alkenes with aqueous hydrogen peroxide. It was demonstrated that the phase-boundary catalyst system required neither stirring to make an emulsion nor addition of a cosolvent to make a homogeneous solution to drive the reaction. However, some basic facts about them remain unclear, such as the question as to where an effective location of the active sites of PBC: is it the external surface of the catalysts or in their pore? In order toelucidate this problem, TS-1, HZSM-5 and zeolite loaded with alkylsilane–covered sulfonic acid in which the location of the active sites mainly inside the pore system are chosen as model catalysts. Catalytic activity of TS-1 and HZSM-5 catalysts were examined after modification with alkylsilane (ODS). Their activities are compared with zeolite loaded with alkylsilane-covered titanium oxide particles, in which the active sites are on their external surface in reactions of 1-octene with aqueous H2O2 and cyclohexene with water as model reactions. It was suggested that the location of the active sites on the external surface plays an important role in ”phase-boundary catalysis” phenomenon.