Mercury Embrittlement is a significant problem in natural gas facilities using aluminum cold boxes and has led to a number of major plant incidents. It has been postulated that the natural passive layer on aluminum provides an effective barrier between droplets of mercury and the underlying aluminum and this prevents mercury embrittlement. However, under certain circumstances this barrier is breached and embrittlement can occur. In this paper the condition of the mercury-aluminum interface is measured using ac impedance techniques and the effects of film aging conditions on the stability of the interface are studied. The film was modeled as a constant phase element and resistor in parallel and it was found that the resistance of the film increased with aging, although the effective film thickness only changed slightly. The paper finds that galvanic corrosion between the mercury droplet and underlying aluminum is likely to be a major cause of the breakdown of the mercury-aluminum interface.