Dielectric spectroscopy is employed in two polystyrene-polyisoprene-polystyrene (SIS) triblock copolymers well below the order-to-disorder transition temperature and in the frequency range from 10(-2) to 10(6) Hz. Small angle X-ray scattering has shown the formation of lamellar structures with a long period of about 25 nm. Besides the polyisoprene and polystyrene segmental relaxations and a slower process associated with the reorientation of the interface, we provide evidence for a new type of chain dynamics associated with the mobility of the junction points at the interface. From the relaxation strength of this process-which is very much reduced as compared to the chain relaxation in bulk polyisoprene-we extract a characteristic length of the end-to-end vector fluctuations in the interface in the range 4-6 nm. This value compares well with an independent estimate of the interfacial thickness based on thermodynamics. Dielectric spectroscopy can therefore be used as a dynamic probe of the interface in ordered triblock copolymers. Over the same temperature range rheology is influenced by a broad spectrum of modes related to the dynamics of tethered polyisoprene chains.