In an effort to improve passenger comfort, the automotive industry is evaluating more sophisticated techniques to reduce engine excited frame vibrations. A new approach is to implement hydraulic mounts in a semi-active mode. In this mode, low power actuators are used to change the impedance characteristics of the mount. This paper presents a dynamic model and analysis of the semi-active hydraulic mount. Experimental data confirming the mount model are presented. Finally, results of an experimental study on the implementation of the semi-active mounts on a production automotive vehicle are presented. The study focuses on frame vibration reduction during engine idle. The semi-active hydraulic mounts show improved vibration levels over passive hydraulic mounts in the vehicle application.