Method for sealing plastic capsules (consisting of at least two telescopically fitting parts) by welding, involves applying energy from a jet of hot gas or a laser beam during relative movement around the vertical axis of the overlapping regions of the capsule parts, such that the capsule material is melted (but not decomposed) in this region and the overlapping regions are securely sealed by a seam region. Independent claims are also included for: (1) capsules welded by the process; (2) a capsule holder for use in the process, consisting of at least two parts which enclose the capsule such that the region to be welded is uncovered to a width of up to 3 mm for application of the radiation; and (3) apparatus for carrying out the process, consisting of apparatus for delivering the laser beam or jet of hot gas, a capsule holder as in (ii) and apparatus (preferably part of an assembly line) for exposing the capsule holder to the energy source.