This paper describes an investigation of free convective flow and thermal structures in an open ended inclined cylindrical thermosyphon with application to an evacuated tubular solar collector. The flow structure was visualised using dye and rheoscopic tracer particles and temperature measurements were made by a traversing thermocouple rake. During steady-state uniform heating a significant stagnant region was observed near the closed end of the tube, the length of which decreased with increasing wall temperature. For the case of differential heating, with the top half of the tube cross section at a higher temperature than the bottom half, there was no stagnant region. When the top half was at a lower temperature than the bottom half, there was a significant region of multicellular motion in the closed end of the tube. An inactive stagnant or multicellular region in an evacuated tube solar collector and other open thermosyphons would decrease the effectiveness of the heat transfer through the open end of the tube.