A general approach, based on the polymer reference interaction site model (PRISM) integral equation theory, suitable for characterizing arbitrarily complex polyolefin melts is described. We tested the method by calculating the melt structures of linear polyethylene (PE) and isotactic polypropylene (iPP) and the spinodal decomposition temperatures for PE/iPP blends. The computational expense of the PRISM calculation was reduced with a single-site united atom model in which the polyolefin CH, CH2, and CH3 groups were approximated as chemically equivalent sites with spherically symmetric energetic interactions. The site-site interactions were defined by a potential function comprising a hard core with an attractive Lennard-Jones term. These energetic parameters were optimized with a central composite design strategy that enabled a simultaneous fit of experimental melt density and structure factor data. Values were obtained for PE and iPP individually and for common universal parameters that could potentially be used for all polyolefins. The rotational isomeric state-metropolis Monte Carlo (RMMC) technique was used to generate sets of conformers at specified temperatures covering the melt-temperature range of the polymers. The characteristic ratio was used to assess the quality of the conformers and the RMMC method. Values of 9.68 for PE and 9.27 for iPP were obtained. The single-chain structure factors calculated by the RMMC method were used to calculate the total structure factor for each melt. These were validated against published X-ray diffraction results.