Natural and synthetic poly(cis-1,4-isoprene) were characterized by size-exclusion chromatography coupled with an online multi-angle light scattering detector (SEC-MALS). Unlike synthetic poly(cis-1,4-isoprene) (SR), natural rubber (NR) samples showed anomalous elution profiles. The beginning of elution was very similar to SR but, after a certain elution volume, the molar masses of the eluting macromolecules increased with elution volume instead of continuing to decrease, which resulted in an upturn curve profile. Adding tetrabutylammonium bromide (TBABr) to THF (solvent and mobile phase) removed this phenomenon. In addition, using different concentrations of TBABr showed that TBABr had two simultaneous actions. TBABr reduced the abnormal elution profiles and the quantity of aggregates (insoluble part or gel). These results mean that the main phenomenon involved in abnormal elution was delayed entities adsorbing on the column packing. Their delayed elution was responsible for the artificial increase in molar masses, especially at high elution volumes. The results obtained suggest that these entities are very compact and have a sphere-like structure.