Chain-level mixing in polyolefins is investigated for blends of polyisobutylene (PIB) and polyethylene-co-1-butene (PEB). Previous reports suggest that PIB exhibits unusual mixing behavior in certain saturated blends relative to other polyolefins, even though it is immiscible with most. Variable-temperature H-1, H-2, C-13, and Xe-129 NMR experiments are used to characterize local PIB chain dynamics in blends with PEB in which the concentration of 1-butene comonomer units is 23 or 66 wt %. Results from 1D and 2D solid-state C-13 exchange experiments, H-1 relaxation measurements, and H-2 line shape analysis indicate that local conformational dynamics of the PIB CH2 group in the polymer backbone increase significantly in blends with PEB copolymers containing 66 wt % butene comonomer (PEB-66). Even though the PEB-66 is a higher T-g polymer than PIB, PIB exhibits a lower effective Tg when the blend is formed relative to its pure state. Similar perturbations are not observed in the PIB/PEB-23 blend, indicating that this blend is not miscible at the chain level. These results are directly relevant to the length scale of glass transitions in polyolefins, indicating that local interchain packing plays an important role in the conformational dynamics that occur within a chain, and are suggestive of local configurational entropy contributions to mixing. Although H-1 spin-diffusion experiments could not reveal quantitative length scales of mixing in the these blends due to insufficient contrast between the constituents, Xe-129 NMR experiments showed that the PIB/PEB-66 blend was homogeneous on a 50 nm length scale. In agreement with the heterogeneous morphology indicated by the dynamic NMR experiments, Xe-129 NMR showed two resolved peaks for the PIB/PEB-23 blend, indicative of phase separation on a 50 nm length scale. The compilation of all the data, most of which was obtained at natural abundance, indicates that the PIB/PEB-66 blend exhibits intimate chain-level mixing on a length scale much shorter than R-g (ca. 8 nm). More importantly, the data show that reduced chain packing constraints occur in the miscible blend and suggest that local entropic contributions are the driving force for miscibility.