A tube-type reactor includes a reactor shell surrounding a bundle of vertically-arranged jacketed tube assemblies. An equilateral triangle is formed by lines interconnecting the central axes of the three jacketed tube assemblies in mutual contact. Each jacketed tube assembly includes a reactor tube forming an annular space within a sheathing tube which includes six longitudinally-arranged slots uniformly distributed about its top end and into which connecting elements are received and welded to join together the sheathing tubes. In one form, the connector element includes spacer heads extending along a web to lie within the annular gap between the reactor tube and the sheathing tube and abut against the reactor tube. In a second embodiment, the connector element includes a web having three spider arms with longitudinal recesses, the width of which corresponds to the wall thickness of a sheathing tube.