Vapor-phase epoxidation of propene using H-2 and O-2 over Au/Ti-MCM-41 with or without a promoter has been investigated at various temperatures and at a space velocity of 4000 h(-1) ml/g (cat). As a promoter, CsCl was impregnated and/or physically (simple mixing) or mechanically (crushed in a mortar) mixed at various concentrations. The support, Ti-MCM-41 (Ti/Si = 3/100), was characterized by XRD, UV-Vis, FT-IR, and specific surface area measurement, whereas Au/Ti-MCM-41 with or without a promoter was characterized by TEM, XPS and ICP techniques before and/or after the catalytic tests. Au/Ti-MCM-41 as such gives propene and H-2 conversions of 3.1 and 47%, respectively, with a PO selectivity of 92% at the reaction temperature of 100 degrees C after 60 min of reaction. Physical mixing of CsCl with Au supported on Ti-MCM-41 reduces H-2 consumption by about 90% and improves propene oxide (PO) selectivity up to 97% at a propene conversion of 1.7%. One constraint is that agglomeration of gold particles is caused by Cl- anions: the mean diameter of Au particles, 2.2 nm, in Au/Ti-MCM-41 increases to about 10-20 nm and some clusters are; even larger than 50 nm in size due to direct contact between chloride and Au particles.