AbstractAn absolute X‐ray method for the determination of the crystalline fraction in polymers based on the comparison of the integrated intensities of some of the low order lines of polymers with those of the completely crystalline standard substances, is described. The crystallinity data are obtained for six polymers: 45.6% for isotactic polypropylene, 73% for polyethylene, 52.1% for nylon‐6, 50.8% for nylon‐66, 32% for isotactic polystyrene, and 33.4% for poly‐4‐methylpentene‐1.These data are in a reasonable agreement with the data obtained by the other X‐ray procedures. The accuracy of the method, however, is restricted by the general impossibility to know precisely the structure factors and the corrections to the intensities due to the thermal vibrations of the atoms and lattice