The infrared spectra of single‐crystal mats of physical mixtures of polyethylene and perdeuteropolyethylene show splittings of CH2and CD2bending and rocking modes which are smaller than those for the pure species. This is believed to be a result of the penetration of cilia from the surface of one crystal into the lattice of an adjacent crystal. Calculations of the concentration of cilia are in good agreement with percentages derived from the study of a mixed‐crystal paraffin system, and these concentrations change with crystal thickness as is predicted. Such cilia can account for most of the amorphous component of polyethylene single crystals.