The oligomerization of isoprene with lithium naphthalene and the dimerization of isoprene with alkali dispersion are described. Isoprene dimer, prepared by the action of lithium naphthalene, is a mixture of 2,6-dimethyl-2,6-octadiene and 2,7-dimethyl-2,6-octadiene, but the trimer, tetramer, and hexamer are a mixture of 3,4-adducts of isoprene to the above mentioned isoprene dimer. The authors have found that alkali dispersion in the presence of a Lewis base is able to dimerize isoprene.