A method of producing an alkylene oxide comprising: passing a reaction mixture comprising alkylene, oxygen and a gaseous chlorine-containing promoter species selected from C1 to C8 chlorohydrocarbons over a supported catalyst comprising silver and rhenium in a range of from 180 ppmw to 2000 ppmw, to undergo an epoxidation reaction at a first operating condition to form an alkylene oxide in a reactor; and subsequently performing the epoxidation reaction within the reactor at a preferred operating condition, wherein the preferred operating condition has an efficiency of the epoxidation reaction toward the alkylene oxide (defined as the percent of reacted alkylene that forms the desired alkylene oxide) which is lower than that of a maximum efficiency achievable at an operating temperature corresponding to the preferred operating condition, characterised in that the preferred operating condition is determined by determining a derivative of efficiency (defined as selectivity or the percentage on a molar basis of converted or reacted alkylene that forms alkylene oxide) to productivity (wherein productivity is expressed as the concentration of alkylene oxide in the reactor outlet stream in mole percent) of the epoxidation reaction as a function of the overall catalyst chloriding effectiveness value (Z*) at a fixed operating temperature, wherein said derivative is in a range of -1 to -4, and wherein the overall catalyst chloriding effectiveness value (Z*) is represented by the following formula: Z * = e t h y l c h l o r i d e equivalent ppmv e t h a n e equivalent m o l e percent wherein the ethyl chloride equivalent is the concentration in ppmv of ethyl chloride that provides substantially the same catalyst chloriding effectiveness of the gaseous chlorine-containing promoter species present in the reaction mixture at the concentrations of the gaseous chlorine-containing promoter species in the reaction mixture; and the ethane equivalent is the concentration of ethane in mole percent that provides substantially the same catalyst dechloriding effectiveness of the non-chloride containing hydrocarbons in the reaction mixture at the concentrations of the non-chloride containing hydrocarbons in the reaction mixture. passing a reaction mixture comprising alkylene, oxygen and a gaseous chlorine-containing promoter species selected from C1 to C8 chlorohydrocarbons over a supported catalyst comprising silver and rhenium in a range of from 180 ppmw to 2000 ppmw, to undergo an epoxidation reaction at a first operating condition to form an alkylene oxide in a reactor; and subsequently performing the epoxidation reaction within the reactor at a preferred operating condition, wherein the preferred operating condition has an efficiency of the epoxidation reaction toward the alkylene oxide (defined as the percent of reacted alkylene that forms the desired alkylene oxide) which is lower than that of a maximum efficiency achievable at an operating temperature corresponding to the preferred operating condition, characterised in that the preferred operating condition is determined by determining a derivative of efficiency (defined as selectivity or the percentage on a molar basis of converted or reacted alkylene that forms alkylene oxide) to productivity (wherein productivity is expressed as the concentration of alkylene oxide in the reactor outlet stream in mole percent) of the epoxidation reaction as a function of the overall catalyst chloriding effectiveness value (Z*) at a fixed operating temperature, wherein said derivative is in a range of -1 to -4, and wherein the overall catalyst chloriding effectiveness value (Z*) is represented by the following formula: Z * = e t h y l c h l o r i d e equivalent ppmv e t h a n e equivalent m o l e percent