A lightweight balloon‐borne grab‐sampling system for determining mixing ratios <10−9of chlorocarbons and other minor constituents in the stratosphere is described. The all‐metal system consists of five sampling spheres and associated valves, which are opened and closed at selected altitudes by on‐board electronics during a parachute descent. The construction features, operating techniques, and experimental tests are given. In the latter, particular attention has been given to the problem of surface adsorption in ambient‐pressureinsitusampling of trace stratospheric constituents at these low mixing ratios.