The isothermal section of the phase diagram of the Co-Ni-Zr ternary system at 1198 K has been investigated by means of diffusion triple and electron microprobe analysis. This isothermal section consists of six threephase regions. The homogeneity ranges of Co23Zr6, Co2Zr, CoZr, Ni5Zr, Ni7Zr2 and NiZr are about 16.7%-18.8 at.%Zr, 28.6%-32.3 at.%Zr, 49.2%-51.7 at.%Zr, 16.0%-18.7 at.%Zr, 22.3%-24.1 at.%Zr and 49.8%-52.1 at.%Zr, respectively; CoZr2 and NiZr2 are nearly linear compounds. The largest olubilities of Ni in Co23Zr6, Co2Zr and CoZr are about 4.9 at.%Ni, 5.4 at.%Ni and 4.5 at.%Ni, respectively. CoZr2 and NiZr2 form a continuous solid solution (Co,Ni)Zr2. No ternary compound is observed.