C-13 NMR analysis revealed the existence of additional unsaturated terminal structures in oligoisobutylene samples, in addition to the well-known -CH2C(CH3)=CH2 and -CH=C(CH3)(2). Structure with a double bond between two quaternary carbons was detected both in commercial Oppanol B3 and in a oligoisobutylene (PIE) sample prepared by polymerization in CH2Cl2 with a high concentration of monomer in the presence of BCl3 at -20 degrees C. In the latter sample, another type of the structure with a double bond between quaternary carbon and methine carbon (-CH=C(CH3)-CH2CH3) was found in a large amount.