Lithium-7 NMR spectroscopy was used to study the dilithium initiator (DLI) (1,3-phenylenebis(3-methyl-1-[methylphenyl]pentylidene)dilithium (1). In hydrocarbon solution, 1 was found to exist predominantly as polyaggregated species, and all the aggregates exchanged rapidly on the NMR time scale at temperatures above 0-degrees-C. Addition of an equivalent per lithium of the tridentate ligand pentamethyldiethylenetriamine (PMDETA) to 1 quantitatively converted the various polyaggregates to a single new species which was assigned to be monomeric 1 with each lithium complexed by one PMDETA ligand. The early stages of isoprene polymerization initiated by both the monolithium initiator (MLI) (1,1-diphenyl-3-methylpentylidene)lithium (2) and 1 were monitored by Li-7 NMR. Several different poly(isoprenyllithium) aggregates were visible in polyisoprene solutions initiated with 2, one of which exchanged much more rapidly with dimeric 2 than the others. Initiation with DLI 1 results in formation of a third type of species along with the two poly(isoprenyllithium) aggregates. This third species contains initiator living ends and is believed to be a mixed aggregate of both isoprenyllithium and initiator. Initiation by both 1 and 2 proceeds more rapidly than subsequent propagation. Under seeding reaction conditions, PMDETA has no obvious effect on the early stages of isoprene polymerization initiated by either 1 or 2.