AbstractA terpolymer of propylene‐1‐butene‐ethylene (TERPO) and a reactor mixture of TERPO with an ethylene‐1‐butene copolymer (BLEND) were completely characterized by TREF, 13C‐NMR, DSC, and GPC, from which special equations for quantitative 13C‐NMR were derived. TERPO was shown to be composed mainly of highly isotactic propene and similar amounts of ethylene and 1‐butene. BLEND fractions were composed of variable amounts of TERPO and a random copolymer of ethylene‐1‐butene. The blend of TERPO and copolymer acts as two independent phases, each having its own elution temperatures dependent only on its crystallizability, itself only influenced by the comonomer content. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 1880–1890, 2001