The binary system between para-dichlorobenzene and para-chloroiodobenzene is presented. The phase diagram shows a peritectic phenomenon at high temperature for concentrations very close to pure para-dichlorobenzene due to the α-β polymorphism of this compound. The main characteristic of this diagram is a syncrystallisation domain in all proportions for the α-phases with a Gibbs' minimum. Crystalline parameters of all the molecular alloys are given. © 1991.