The solid acid-base catalysts comprising Of SiO2-X-m-P-n-O-p (X: alkali metal, alkaline earth metal) showed high catalytic activity and selectivity in vapour phase intramolecular dehydration of monoethanolamine (MEA) to ethylenimine (EI). These catalysts have acid sites and basic sites that were controlled extremely week (+4.8 < H-0, H- < +9.3). On the basis of the results obtained from pulse reaction carried out with these catalysts and TPD/IR measurements of the catalysts, the following mechanism is concluded. MEA is preferentially adsorbed from the side of the hydroxyl group, which is then dissociated on acid and base sites of the catalyst simultaneously, and the consecutive dehydration occurs to form EI. The newly developed vapour phase process using these catalysts has been operated in the commercial plant since 1990.