The isothermal cross-section through the ternary phase diagram Co–Ni–Si at 800°C was constructed. No ternary phases exist in the system at this temperature. The topology of the Co–Ni–Si isotherm is determined by the formation of continuous solid solutions between isomorphous Co2Si and Ni2Si intermetallics and cubic CoSi2 and NiSi2 phases and by rather extensive cobalt/nickel exchanges in other binary silicides. The CoSi intermetallic compound was found to be a dominant growing phase in the binary Co/Si as well as ternary Co–Ni/Si couples with the initial end-members containing up to 50 at.% of Ni. When the Co–Ni solid solution with 30–70 at.% of nickel was used, the formation of NiSi2 and NiSi (instead of CoSi2) intermetallics next to the Si-substrate was observed after interaction at 800°C.