An experimental study has been conducted to validate theoretical solutions for the response of a base excited single degree of freedom isolation system possessing pure cubic damping. The cubic damping characteristic was implemented using an electromagnetic shaker with a simple nonlinear velocity feedback control. The rig and practical implementation of the active damping are described. The base excitation was harmonic at a set of discrete frequencies with constant amplitude. Consistent with theoretical predictions, the isolation performance at high excitation frequencies is shown to be worse than either the undamped or linearly damped isolation system with the displacement transmissibility tending to unity. This is contrary to the case of force excitation reported in the literature where cubic damping offers improved performance. The physical causes of the distinct behaviours and the consequences for isolator design are discussed.