The complete miscible solid solution with a minimum freezing point (-50.75℃; 26.88% para-) of the ortho- and para-chlorotoluene has been proved in this paper by many methods as the thermal analysis method, the X-ray matter phase analysis, the fractional crystallized separation method and the solidified separation method of solid solution, and by the Fel'dman's experiential productivity formula compared with the theoretical productivity formula which was obtained from the solidified separation according to the solid solution phase diagram. Ther...