A study was made of cationic polymerization of the cis- and trans-isomers of penta-1,3-diene. The microstructure of polymer chains was studied by NMR, IR spectroscopy and ozonolysis. It was shown that, irrespective of the type of penta-1,3-diene isomer, trans-1,4 and trans-1,2 units represent the linear unsaturation of polymer chains. According to the overall unsaturation of polymers, which is 62–92% of theory, the number of trans-(1,2+1,4) units varies between 45 and 80% and of 1,2-units, between 11 and 26%. The content of 3,4-units may reach 6·5%. It was shown that cyclic fragments of polymer chains are monocyclic cyclohexane structures containing a tetra-substituted double bond. Views have been expressed about the features of cationic polymerization of penta-1,3-diene isomers.