The effect of additives and seeding on the molecular weight distribution of diene polymers prepared by two hydrocarbon-soluble dilithium initiators, 1,3-phenylenebis(3-methyl-1-phenylpentylidene)dilithium and 1,3-phenylenebis[3-methyl-1-(methylphenyl)pentylidene]dilithium, was investigated. Without any additives or seeding, the distribution of polyisoprene was bimodal and broad by the standard of polymers prepared by anionic polymerization. A particularly effective additive was N,N,N",N",N"-pentamethyldiethylenetriamine (PMDETA). Distribution with M(w)/M(n) of 1.07 was obtained with the microstructure of the polyisoprene only slightly affected. Several lithium alkoxides narrowed the distribution but not as effectively as PMDETA. Lithium alkoxides, however, kept the microstructure of the polyisoprene unchanged. Another tertiary amine, N,N,N’,N’-tetramethylenediamine, was also less effective than PMDETA. Seeding, by reacting the initiator with a fractional amount of isoprene first and then with the rest of the monomer, also made narrow distribution polyisoprene. The distributions of polybutadiene prepared by the dilithium initiators were also made narrower by the same techniques.