A simple and straightforward demonstration of the temperature dependence of partial miscibility is described. The Gibbs phase rule, temperature–composition phase diagrams, and the lever rule serve as the basis for interpretation of the demonstration. The water–triethylamine system is miscible in all mixing ratios at temperatures less than 18 °C, but separates into two immiscible phases for a wide range of bulk compositions as temperature increases through 19–24 °C. A procedure is described in which a range of bulk compositions simultaneously experience an increase in temperature. Students are able to see differences in the temperature of phase separation, plus the evolving compositions and volumes of the separated phases.