A series of linear 1, 2 and 3, 4 polyisoprenes were cyclized to various degrees by the action of TiCl4 and POCl3 as catalysts. Light scattering and viscometric studies of cyclized samples showed that the cyclization process was accompanied by an increase of the steric hindrance factor σ up to a certain value. At higher degrees of cyclization, on the other hand, the characteristic dimensions of the macromolecules diminished due to the internal cross‐linking.