A structural characterization of a propylene-styrene-ethylene terpolymer sample (sP/(S-E)), prepared with syndiospecific C-s-symmetric metallocene catalyst, is presented. The study of the polymorphic behavior in as-prepared, melt-crystallized, and fiber samples has confirmed that ethylene units are mainly bound to styrene units arising from catalyst reactivation after the secondary insertion of styrene. Styrene-ethylene units are mainly segregated in the amorphous phase. The bulky styrene units, bound to ethylene, prevent the inclusion of ethylene units in the crystals of syndiotactic polypropylene (sPP), as generally occurs for copolymers of sPP with ethylene. This explains the experimental evidence that the influence of the presence of styrene-ethylene units on the polymorphism of sPP is smaller than that observed in copolymers of sPP containing only ethylene or butene comonomeric units.