A new instrument for the determination of fine temperature profiles in hot, corrosive, electrically conductive, unsteady two‐phase fluid systems under vacuum or pressure is described. It features simultaneous measurement of temperatures at close locations, by a column of miniature (0.25 mm diam) thermistors mounted at the tips of supporting hypodermic tubes, and streamlined design that minimizes hydrodynamic and thermal disturbances at the measured locations. Some of the techniques used in the construction, such as the welding, insulation, and handling of fine wires (0.025 mm diam), are of interest in many other applications. The instrument has been used to measure temperature distributions with an accuracy of ±0.02°C in a model of a flash‐evaporator stage used for the desalination of saline water, and thus provided new and quantitatively significant data about the process.