Abstract: A reactor for single-layer, exothermic catalytic synthesis in the gaseous phase under pressure, is made up of a main casing under high pressure, comprising a cylindrical part and two hemispherical ends containing an enclosure for the catalyst carrier, including a single catalyst layer concentric to the body of force and whose external wall is covered with an insulator, a space being located between the internal wall of the main casing under high pressure resting on a helical metal winding from one end of the casing to the other, said main casing under high pressure and the enclosure for the catalyst carrier being provided with means for introducing and withdrawing gas streams, located at the ends of the main axis and arranged facing each other on the hemispherical ends. The enclosure for the catalyst carrier furthermore is equipped with a vertical pipe for filling and emptying the catalyst. The reactor can be used for synthesizing ammonia and methanol.