A complete solid‐solution series exists between diopside (CaMgSi2O6) and its nickel analogue, “niopside”(CaNiSi2O6). Activity–composition relations within this solid solution, and the stability of the end member CaNiSi2O6, have been determined by equilibrating CaNiSi2O6with SiO2, CaSiO3, and metallic Ni in atmospheres of known oxygen pressures. Within limits of accuracy of the experiments, the solution is ideal at 1350°C. From the experimental data obtained in the present investigation, the standard free energy (ΔG°) of formation of CaNiSi2O6according to the equation CaO + NiO + 2SiO2= CaNiSi2O6is calculated to be ΔG°=−165862 + 42.40TJ. Experiments in the system CaO–NiO–SiO2have shown that the nickel analogue of the phase pseudo‐enstatite (MgSiO3) is unstable with respect to SiO2and nickel olivine (Ni2SiO4), and the nickel analogues of the phases akermanite (Ca2MgSi2O7) and monticellite (CaMgSiO4) are unstable relative to the phase assemblage pseudo‐wollastonite (CaSiO3) plus NiO. In the system CaO–MgO–NiO–SiO2, however, substitution of Ni for Mg in these phases was observed. The percentage substitution of Ni for Mg in the phases is given in parentheses: diopside (100%), olivine (100%), enstatite (18%), akermanite (