Antimony vapor pressures were determined in the ternary In-Ni-Sb system along, an isopleth with x(In)/x(Ni)=1/9, using an isopiestic method. The measurements were made between 980 and 1350 K, covering a composition range 43-48 at% Sb, with a few samples at higher antimony contents. Thermodynamic activities and partial molar enthalpies of antimony were derived for the ternary zeta-phase which crystallizes in an NiAs-type structure. The composition dependence of the activities is given for a temperature of 1173 K, and isoactivity curves are constructed for the same temperature. The variation of the antimony activity along the zeta/(zeta+L) phase boundary is estimated.