Polymer reference interaction site model (PRISM) calculations and wide-angle X-ray experiments were performed on vinyl polyolefin melts of polyethylene (PE), isotactic polypropylene (iPP), and poly(I-butene) (P1B). PRISM calculations were also carried out on poly(1-hexene) (P1H). For PE and iPP melts a single main peak in the scattering pattern was observed near wave vectors k similar to 1 angstrom(-1); for P1B and P1H we observed this main peak to split with the development of a prepeak that moves to lower wave vectors as the side chain size increased. By calculation of the various partial structure factors, we found that the prepeak formation is due to a combination of two effects: (1) an increase in the average distance between the chain backbones due to screening of the side chains and (2) increased intermolecular ordering of the pendant methyl groups as the side chains become longer. The scattering of random copolymers (PEP, PEB, PEH) of PE with iPP, P1B, and P1H, respectively, was also studied with PRISM theory as a function of composition. Development of a prepeak in the scattering from PEB and PEH was predicted to occur at copolymer compositions of less than 50% PE. No prepeak was predicted to occur at any composition in the PEP copolymer.