•Self-rewetting fluids can enhance capillary evaporator performance.•The phase change is visualized.•The enhancement of capillary evaporator with self-rewetting fluids is explained. This paper presents an experimental study of the phase change phenomenon in a porous medium. The tested working fluids are pure water and butanol aqueous solutions with different concentrations. In contrast to ordinary fluids, the surface tension of self-rewetting fluids exhibits a positive gradient beyond a certain temperature value. The experimental results indicate that the use of self-rewetting fluids (water/butanol) as working fluid significantly improves the performance of the capillary evaporator by decreasing the casing temperature. To explain the heat transfer enhancement mechanism, the phase change phenomenon is visualized for the two working fluids. It is shown that as the applied power increases, the shape of the vapor pocket that developed within the porous wick also increases for pure water until it reaches a stable shape. With respect to self-rewetting fluids, the shape of the vapor pocket decreases with increasing applied power allowing more efficient mass and heat transfers. Wettability, capillary pressure and Marangoni forces are the factors related to surface tension and contact angle that seem to be responsible for this heat transfer improvement for self-rewetting fluids.