Highly pure dendritic single crystals of cadmium iodide, variously (2 to 8%) doped with lead iodide, have been grown from the vapour phase in a vacuum. The study of polytypism and related phenomena in the crystals has been carried out by X‐ray diffraction. An uneven high rate of increase in the occurrence of the 12‐layered rhombohedral polytype 12Rhas been found to exist when the amount of PbI2in the crystals rises from 2 to 8%. Simultaneously, the relative intensity of the 12Rreflections has been found to rise rapidly. Another curious feature has been an unusually high frequency of occurrence of a particular high polytype 48Hin the heavily (8%) doped crystals. The percentage occurrences of streaking and arcing have been found to be low and to vary slowly with the PbI2content of the crystals. The observed polytypism has been explained in terms of the nucleation of two different thermodynamically stable end‐member polytypes, 4Hand 12R, of CdI2and PbI2, respect