Monomer sequence distribution of copolymers of butadiene and isoprene prepared with lanthanide complexes LaCl2-C2H5OH-Al(C2H5)2 was determined by 13C-NMR spec-trometry.The dyad distribution and number-average sequence length of these copolymers were quantitatively determined. The results show that sequence distribution of monomer units in copolymers prepared with Nd

Pr and Ce catalysts follow Bernoullian statistical model

while copolymers prepared with Y

Sm

Dy and Gd catalysts show some deviation from it.