The determination of the solid-liquid phase diagram of a binary system is always used as an experiment in the undergraduate physical chemistry laboratory courses. However, most phase diagrams investigated in the lab are simple eutectic ones, despite the fact that complex binary solid-liquid phase diagrams are more common. In this article, the cooling curves of phenol-t-butanol mixtures are measured and the phase diagram of this system is found to give three eutectic points and two congruent melting points. The laboratory provides the students a practical way to investigate a complex system with their own efforts and teamwork spirit.