The heat capacities of p-terphenyl-based crystals that are mixtures with 3, 6-diphenyl-1, 2, 4, 5-tetrazine have been measured by adiabatic calorimetry below room temperature. The concentration dependence of the apparent entropy of transition for the twist transition of the host p-terphenyl crystal is examined, and clearly shows that the phase transition is of order-disorder type, where the conformation of an individual molecule plays the role of an Ising spin variable.