A new technique has been developed for locating air leaks in vacuum devices. It employs a sealed‐off, highly evacuated ionization gauge having a section of its envelope made of thin palladium sheet which, when heated, is highly permeable to hydrogen. In use the gauge is connected to the manifold of a vacuum system with the palladium section isolating the vacuum of the gauge from that of the manifold. When the manifold or any vacuum device connected to it is probed with hydrogen, leaks in the system are indicated by an increase in the ion current of the gauge. This new technique, because it utilizes a sealed‐off, highly evacuated gauge which responds only to hydrogen and is unaffected by other residual gases and vapors, provides a simple but sensitive test for very small leaks.