Rotor vibration attenuation and structural components isolation in jet engines are achieved with squeeze film dampers, many of them supported on long elastic squirrel cages. Integral squeeze film dampers (ISFDs) are comprised of arcuate pads and wire-EDM webs rendering a compact viscoelastic support. An experimental study is conducted to evaluate the effectiveness of ISFDs in attenuating the imbalance response of a massive test rotor. Measurements of the damper structural stiffness and rotor natural frequencies are detailed. Impact tests on the test rotor supported on its dampers reveal the supporting structure to be very flexible, thus requiring the experimental evaluation of an equivalent stiffness for the damper and supports system. System damping coefficients extracted from impact load excitations vary with the lubricant viscosity and include a significant structural damping from the bearing supports. Rotor coast-down tests demonstrate the ISFDs to damp well the rotor response with peak vibration amplitude proportional (linear) to the imbalance. Viscous damping coefficients estimated from the amplitude response at the critical speeds agree reasonably well with predictions from a full-film, finite element model.