Four copolymer samples of propylene with a small amount of butene were prepared by different MgCl2-supported Ziegler-Natta catalysts and were fractionated by using the preparative temperature rising elution fractionation (TREF) technique. Some selected fractions were characterized by C-13 NMR. The data obtained suggest that the fractionation is mainly conducted according to tacticity. The influence of electron donors on the tacticity distribution and the composition distribution was investigated. It is found that both internal and external donors increase the isotacticity and narrow the tacticity distribution of copolymers but do not narrow the composition distribution of copolymers, though the butene content is reduced. C-13-NMR results show that electron donors have a larger effect on the composition of the fractions of highest and lowest isotacticity, leading to a different dependence of composition on elution temperature from that of tacticity. This further confirms the supposition proposed in our previous work that electron donors can be present in the environment of both aspecific and isospecific active sites. The differences between internal and external donors were also compared. It is observed that internal and external donors change the proportion of fractions differently and that an external donor decreases the butene content of fractions more effectively.