A process of reducing catalyst selectivity loss rates in the production of ethylene oxide, which process comprises: reacting a feed comprising ethylene and oxygen in the presence of a catalyst bed comprising catalyst loaded in a reactor tube of an EO reactor containing multiple EO reactor tubes, an EO removal section, a CO 2 absorber section, and a CO 2 removal section, the catalyst comprising silver, cesium, rhenium or compound thereof, and a rhenium co-promoter selected from one or more of lithium, tungsten, molybdenum, chromium, sulfur, phosphorus, boron, and compounds thereof; the process characterised by: (i) determining a ratio of the partial pressure of water (PPH 2 O) divided by the vapour pressure of water (VPH 2 O) at an axial position (z) in the catalyst bed; and (ii) controlling the presence of water in the EO reactor by utilising one or more of the following epoxidation process steps: a) increasing cooling of the overhead streams coming from the EO removal section and/or CO 2 removal sections of the plant that return to the EO reactor; b) diverting less of the recycle gas through the CO 2 absorber section; c) operating the EO and CO 2 absorber sections at lower temperature; d) increasing the Gas Hourly Space Velocity at fixed EO production to reduce the water concentration gradient increase in the EO reactor; e) reducing EO production per unit volume of catalyst to reduce the amount of H 2 O formed in the EO reactor; and f) operating the reactor at higher temperature than required to increase the vapour pressure of water in the EO reactor tubes; so that the ratio of the partial pressure of water (PPH 2 O) divided by the vapour pressure of water (VPH 2 O) at the axial position (z) is less than 0.004. reacting a feed comprising ethylene and oxygen in the presence of a catalyst bed comprising catalyst loaded in a reactor tube of an EO reactor containing multiple EO reactor tubes, an EO removal section, a CO 2 absorber section, and a CO 2 removal section, the catalyst comprising silver, cesium, rhenium or compound thereof, and a rhenium co-promoter selected from one or more of lithium, tungsten, molybdenum, chromium, sulfur, phosphorus, boron, and compounds thereof; the process characterised by: (i) determining a ratio of the partial pressure of water (PPH 2 O) divided by the vapour pressure of water (VPH 2 O) at an axial position (z) in the catalyst bed; and (ii) controlling the presence of water in the EO reactor by utilising one or more of the following epoxidation process steps: a) increasing cooling of the overhead streams coming from the EO removal section and/or CO 2 removal sections of the plant that return to the EO reactor; b) diverting less of the recycle gas through the CO 2 absorber section; c) operating the EO and CO 2 absorber sections at lower temperature; d) increasing the Gas Hourly Space Velocity at fixed EO production to reduce the water concentration gradient increase in the EO reactor; e) reducing EO production per unit volume of catalyst to reduce the amount of H 2 O formed in the EO reactor; and f) operating the reactor at higher temperature than required to increase the vapour pressure of water in the EO reactor tubes; (i) determining a ratio of the partial pressure of water (PPH 2 O) divided by the vapour pressure of water (VPH 2 O) at an axial position (z) in the catalyst bed; and (ii) controlling the presence of water in the EO reactor by utilising one or more of the following epoxidation process steps: a) increasing cooling of the overhead streams coming from the EO removal section and/or CO 2 removal sections of the plant that return to the EO reactor; b) diverting less of the recycle gas through the CO 2 absorber section; c) operating the EO and CO 2 absorber sections at lower temperature; d) increasing the Gas Hourly Space Velocity at fixed EO production to reduce the water concentration gradient increase in the EO reactor; e) reducing EO production per unit volume of catalyst to reduce the amount of H 2 O formed in the EO reactor; and f) operating the reactor at higher temperature than required to increase the vapour pressure of water in the EO reactor tubes; a) increasing cooling of the overhead streams coming from the EO removal section and/or CO 2 removal sections of the plant that return to the EO reactor; b) diverting less of the recycle gas through the CO 2 absorber section; c) operating the EO and CO 2 absorber sections at lower temperature; d) increasing the Gas Hourly Space Velocity at fixed EO production to reduce the water concentration gradient increase in the EO reactor; e) reducing EO production per unit volume of catalyst to reduce the amount of H 2 O formed in the EO reactor; and f) operating the reactor at higher temperature than required to increase the vapour pressure of water in the EO reactor tubes; so that the ratio of the partial pressure of water (PPH 2 O) divided by the vapour pressure of water (VPH 2 O) at the axial position (z) is less than 0.004.