The structural characterization, by X-ray diffraction and solid-state C-13 n.m.r. CPMAS, of copolymers of propylene with small amounts of ethylene and 1-butene, synthesized with a single centre syndiospecific catalyst, is presented. The effect of the presence of comonomeric units on the polymorphic behaviour of syndiotactic polypropylene is discussed. In the as-prepared samples, the presence of small amounts of ethylene induces the crystallization of conformationally disordered modifications of form II, intermediate between the limit ordered forms II and IV. The conformational disorder is characterized by the presence of long trans-planar sequences, including the ethylene units, and gives rise a structure with kink bands. In copolymers with l-butene the usual crystallization in form I occurs. In samples of copolymers crystallized from the melt, the usual crystallization of form I is not affected by the presence of any comonomeric units, although disordered modifications of form I are always obtained.