The apparatus described here permits the study of phase diagrams either by heating or cooling. One temperature control circuit permits a preset temperature gradient, either positive or negative, to be maintained between the sample and bath with the temperature of the bath being controlled by that of the sample. The appearance or disappearance of solids in the sample is followed by measurement of the light transmission of the sample. A second control circuit provides constant bath temperatures. A step‐wise approach to the liquidus point, with the light transmission used to indicate the establishment of equilibrium between solid and liquid at each step, offers a means of determining this point under equilibrium conditions.