Results are presented for a laboratory study of a compact, single-axis hybrid (active-passive) vibration isolator. The passive system component demonstrates a very high level of vibration isolation at frequencies roughly a factor of 3 above the fundamental system resonance. The active component complements the system by significantly reducing the transmitted vibration levels at lower frequencies, where the passive-only system is ineffective. The device consists of three basic components, a passive compliant spring, force and velocity sensing, and a piezoelectric actuation layer. The experimental system is typically excited at 50 Hz with response characteristics measured over the band from ∼ 10 to 2000 Hz. The isolation performance is evaluated for an optimized passive stage as well as for all relevant hybrid layer configurations. The optimal physical control law is determined by identifying positions in the device stack where actuation and sensing are most effective at minimizing the downstream base velocity and power flow through the mount. Local force and velocity minimization are implemented via a least mean squared adaptive control filter. The role of harmonic distortion and actuator nonlinearity is examined by comparing the system performance and out-of-band enhancement obtained using PZT-4, PZT-5H, and single-crystal PMN-PT actuator materials.