The surface tension and density of Ni-based superalloys RENE N5(A (R)) and RENE 90(A (R)) have been measured by the pinned drop method at temperatures ranging from 1638 to 1780 K. In order to obtain accurate reliable data, the tests have been performed under a reducing atmosphere to minimize the oxygen contamination. In the temperature ranges investigated, both properties show a linear temperature dependence. With the aim of evaluating the reliability of the data, the present results have been analyzed using different thermodynamic models. The new experimental data for RENE N5(A (R)) and RENE 90(A (R)) superalloys were also compared with the corresponding data obtained by different experimental techniques.