We present a combined 1H-NMR and small angle neutron scattering in situ study of the anionic polymerization of butadiene using t-butyllithium as the initiator. Both initiation and propagation phases were explored. This combined approach allows the structural and kinetic characteristics to be accessed and cross compared. The use of the D22 instrument (ILL Grenoble) permits the attainment of Q approximately equal to 2 x 10(-3) A. This, in turn, led to the identification of coexisting large-scale and smaller aggregates during all stages of the polymerization. The smaller aggregates contain most of the reacted monomers. Their structure changes from high functionality wormlike chains at early stages of the reaction to starlike aggregates where the crossover occurs at a degree of polymerization of approximately equal to 40. The initiation event involved these small, high functionality (approximately equal to 120) aggregates that apparently consisted of cross-associated t-butyllithium with the newly formed allylic-lithium head groups. As the initiation event progressed the initiation rate increased while the functionality of these small aggregates decreased and their size increased. Propagation, in the absence of initiation, was found to have a rate constant that was molecular weight dependent. At approximately 11 kg/mol the measured polymerization rate was found to increase while no further structural changes were seen.