Silver catalysts that have been modified by Cl addition are more stable with respect to thermal runaway during reaction and exhibit higher activity and selectivity for formation of 3,4-epoxy-1-butene. The Cl effect for activity enhancement is most likely electronic in nature and involves subsurface Cl, although a similar role for surface Cl cannot be excluded. Selectivity enhancement and thermal stability may be due to electronic effects as well, but could also be the result of Ag-site blockage on the surface of the catalyst that lowers the rate of combustion of adsorbed EpB. Unlike alkali promoters used for olefin epoxidation, Cl exists in a very dynamic state. Chlorine is continuously removed from the Ag surface by reaction with EpB and paraffin diluents (that are added to the reaction feedstream). Thus, organic chlorides in the ppm range are also continuously added to the feedstream to replace the Cl lost during reaction. Chlorine is deposited by the dehydrohalogenation of organic halides at the surface of the Ag catalyst. The deposition of surface Cl on the Ag surface is a function of the concentration and reactivity of the organic halide in the feedstream, as well as the concentration of vacant Ag sites. The surface Cl is in equilibrium with subsurface Cl, and the concentration of surface Cl controls the concentration of subsurface Cl. Thus, it is possible to chlorinate a Ag catalyst to levels considerably greater than one monolayer equivalent, based on exposed Ag surface, since the Ag subsurface is capable of "storing" substantial amounts of Cl. The excess Cl can be easily removed by reaction with paraffin reaction diluents, such as n-butane, to give an optimally Cl-modified catalyst. However, silver catalysts can also be overchlorinated by exposure to excessively high levels of organic halide and/or temperature, leading to the formation of bulk AgCl, which is catalytically inactive. Such catalysts cannot be regenerated by Cl removal. (C) 2004 Elsevier Inc. All rights reserved.