By reacting sec. butyl lithium with propylene at 60-65°, in the presence of isopropyl ether or petroleum ether or liquid propylene, 1-lithium-2,3-dimethylpentane isomers free) with 20% maximum conversion has been recovered. In these conditions the addition takes place therefore according to Markownikof's rule, as it is known in literature, for the addition of $\alpha$-aliphatic olefins to beryllium- and aluminium alkyls.