Small angle neutron scattering was used to in situ study the intermediate structures formed in the course of the polymerization of butadiene and isoprene in deuterated n-heptane. These in situ measurements showed that the start of the butadiene polymerization was accompanied by the presence of highly aggregated large-scale structures. The late stages of the isoprene polymerisation, on the other hand, can be well described by assuming a constant aggregation number f=4.