In various industrial applications of closed two-phase thermosyphons, the counter-current flow limitation (CCFL) is the limiting factor for the thermosyphon performance. In this context, liquid-vapour flow separators can be of great importance, since they allow to shift the CCFL towards higher performances. Among the different separator designs that have been proposed, the cross-over separator is very promising; it allows perfect phase separation by guiding the counter-current liquid and vapour flows in separate channels. This paper illustrates the performance characteristics of a closed two-phase thermosyphon with installed cross-over separator. The results of visualization experiments are presented and a model for calculating the required minimum fill ratio is introduced.