In ethynylbenzene, elevated pressure favors cooperative equivalent to CH center dot center dot center dot Pi(C equivalent to C) hydrogen bonds compared to equivalent to CH center dot center dot center dot Pi(arene) and leads to the ordered polymorph nearly isostructural form with the crystals frozen by cooling. The high-pressure polymorph is more resistant to polymerization than the liquid and, in this respect, is a manifestation of Le Chatelier principle explained at the microscopic level for this compound. A new type of the polymorphic relation has been described.