The phase structure of random copolymers of ethylene and ethylene-d(4) with 1-octadecene and other l-alkenes has been investigated. CPMAS C-13 NMR spectra show that a fraction of the central sections of C16H33 side chains in ethylene-d(4) copolymers are in ordered environments at 298 K. They give rise to resonances from 32.9 ppm to 33.8 ppm, which show that they are in trans conformations; T-1C values for this group of resonances range from 1 s to 7 s. The remaining side chains are in an amorphous environment, the internal methylenes having a chemical shift of 30.8 ppm and a T-1C close to 0.4 s. A Raman band at 1062 cm(-1) in the spectrum of an ethylene-d(4)-1-octadecene copolymer is consistent with partial crystallization of side chains. Some side-chain crystallization also occurs in a 1-tetradecene copolymer. X-ray diffraction studies suggest that smaller side chains do not crystallize to a significant extent at 298 K.