Abstract: A fluid filled vibration isolator having a pair of chambers separated by a partition in which is formed an inertia track passageway cooperable with a decoupler to provide certain dynamic operating characteristics is provided with a means for varying the compliance of one of the chambers to enable the dynamic operating characteristics to be precisely adjusted in the course of operation. In the embodiment disclosed, an air chamber is superimposed above a fluid chamber and is separated therefrom by a diaphragm movable into the chamber in response to a differential control pressure, such as a vacuum, applied therein. A shutter panel is mounted to the underside of the diaphragm and moved therewith into and out of engagement with a rigid stop surrounding the diaphragm. When disengaged from the stop, the air chamber contributes to the overall compliance of the fluid chamber.