We studied the deactivation of silica-alkaline-phosphate catalyst for ethylenimine production by vapor phase intramotecular dehydration of monoethanolamine. There are two types of catalyst deactivation-a short time-range deactivation caused by coking and a long time-range deactivation due to structural changes in the catalyst such as sintering or loss of active component. Catalytic activity decreases along with lowering of the surface area when sintering occurs. Sintering is accelerated by water vapor at high temperature. Loss of phosphorus, which is one of the catalytically active components, also causes deactivation. The lost phosphorus in the catalyst can be resupplied by treatment with volatile phosphorus compounds in the reactor, and catalytic activity and selectivity can be recovered. (C) 2007 Elsevier B.V. All rights reserved.