Several aromatic diacetylenes were heated in molten state, and the formation of diradicals was studied by ESR spectroscopy. Intense ESR signals were observed even at temperatures as high as 180 degrees C. The formation of radical species at such low temperatures was explained by the formation of oligomeric species. The numbers of unpaired electrons in the molten state reached to 10(21) electrons per mole of diacetylene, when heated to 180 degrees C, and deep red materials were obtained. Surprisingly the radicals did not disappear when exposed to air showing that the radicals are stable in air.