The miscibility behavior of blends of high molecular weight isotactic poly(propylene) (it-PP) and isotactic poly(1-butene) (it-P1B) has been investigated by a combination of optical and scanning electron microscopies, differential scanning calorimetry, and dynamic mechanical analysis. The results of these investigations demonstrate that for the molecular weights investigated, it-PP and it-P1B form blends that are partially miscible in the liquid state. Liquid-liquid demixing is observed by optical microscopy at temperatures above the melting temperature of the it-PP component and is also inferred from scanning electron micrographs of the freeze fracture surface of quenched blends after extraction of the it-P1B component with cyclohexane. It-PP spherulites grow through both liquid phases at relative rates that depend markedly on the crystallization temperature. The complex multiple-melting behavior of the isotactic poly(propylene) component in the blend is explained in terms of a bimodal distribution of it-PP lamellar crystals which results from crystal growth in the phase-separated liquid. The heterogeneous nature of the mixed liquid is consistent with the upper critical solution temperature behavior observed in our previous study of blends of atactic poly(propylene) and atactic poly(1-butene). Finally, the dynamic mechanical analysis data are explained in terms of a liquid-liquid demixing process that results in a significant degree of phase mixing.