endo-Dicyclopentadiene (DCPD) was polymerized by various cationic initiating systems in different solvents. IR and H-1 NMR results show that four types of structural units are formed due to the corresponding addition modes : the addition on the norbornenic (NB) double bond generates unit I and rearranged unit II, while the addition on the cyclopentenic (CP) double bond produces unit III and rearranged unit IV. The reaction medium has a stronger effect on the microstructure of the polymer (PDCPD) than initiating systems. The polymers prepared in toluene and n-hexane contain all four structural units, while the polymer produced in methylene chloride is composed of structural units II and IV.