Density and surface tension of liquid Cu–Fe–Ni alloys have been measured in an electromagnetic levitator over a wide temperature range, including the undercooled regime. Both properties are linear functions of temperature. Their concentration dependence, however, is highly non-linear. The fit of the density data requires an excess volume containing a substantial ternary contribution. The surface tension is correctly predicted by the Butler equation from the thermodynamic potentials of the binary phases alone. In addition, a simple model is proposed which describes the surface tension reasonably well and requires as input the surface tensions of the pure components only.