We document the presence of solitary structures in the electric field, measured by the Cassini plasma wave instrument at an interplanetary shock associated with the October/November 2003 solar flares. The occurrence frequency of electrostatic solitary waves increases prior to and during the passage of the initial shock boundary but decreases to almost zero in the post-shock environment. The electric field amplitudes of the solitary structures are on the order of a few tens of ¿V/m, while the characteristic scale size is estimated to be ~500 Debye lengths. The estimated potentials are ~0.5 V both upstream and downstream of the shock. These measurements present a new plasma regime which support electrostatic solitary structures.