A terpolymer of propylene-l-butene-ethylene (TERPO) and a reactor mixture of TERPO with an ethylene-l-butene copolymer (BLEND) were completely characterized by TREF, C-13-NMR, DSC, and GPC, from which special equations for quantitative C-13-NMR were derived. TERPO was shown to be composed mainly of highly isotactic propene and similar amounts of ethylene and l-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.