A rapid method for determining isothermal sections of ternary‐alloy diagrams has been developed. Three elements are evaporated simultaneously onto a heated substrate in such a way that the composition of the deposit varies from point to point as it does in a ternary‐phase diagram. An isothermal section of the Fe–Cr–Ni system prepared by the new technique is compared with one reported in the literature. The applicability of this technique to the preparation of ternary phase diagrams is discussed.