In the recent years, the interest towards the application of two-phase thermosyphons as an element of heat recovery systems has significantly increased. The application of thermosyphons is steadily gaining popularity in a wide range of industries and energy solutions. In the present study, a 2-D numerical modelling of a two-phase gas/liquid flow and the simultaneously ongoing processes of evaporation and condensation in a thermosyphon is presented. The technique volume of fluid was used for the modelling of the interaction between the liquid and gas phases. The operation of a finned tubes thermosyphon was studied at several typical operating modes. A parametric study over a non-ribbed and finned tubes thermosyphon was carried out. The commercial software ANSYS FLUENT 14.0 was used for the numerical analysis. It was proven that the numerical modelling procedure adequately recreates the ongoing flow, heat and mass transfer processes in the thermosyphon. The numerical result from the phase interaction in the thermosyphon was visualized. Otherwise, such visualization is difficult to achieve when only using empirical models or laboratory experiments. In conclusion, it is shown that numerical modelling is a useful tool for studying and better understanding of the phase changes and heat and mass transfer in a thermosyphon operation.