The present invention relates to thermosyphons, in particular for use in the cooling of electronic components. In a first embodiment a thermosyphon is manufactured by extruding a base (1) and milling a channel structure in the base to produce a plurality of fins (5) extending vertically from the base. A lid (3) comprising a number of fins (4) extending vertically from the lid is placed over the heat sink channel structure so that a thermosyphon of an expanding channel system is formed. In a second embodiment the evaporator and condenser sections are separated and connected by pipes. To form a leak proof seal between the lid and the base, joining is preferably done by friction stir welding. By providing an extruded thermosyphon, heat transfer is made more efficient than when junctions are used. The present invention provides a new way of efficiently manufacturing an integrated structure, while keeping the heat transfer of the structure high.