An investigation was conducted to: Determine the mechanism of void-gas stripping in a moving packed bed of solids; determine the effect of system parameters on void-gas stripping requirements; and to develop a model for predicting the minimum amount of stripping gas required for a particular design application. The void-gas stripping investigation was carried out with an air-nitrogen gas system. Stripping was conducted in 4, 6, and 8-in. diameter columns with moving beds of sand, siderite, and coal char. The solids flow rate was varied over a wide range. In each test, the stripping gas (i.e., nitrogen) was added to the standpipe to displace the interstitial oxygen in the moving bed of solids. The oxygen concentration in the standpipe was monitored with a diffusion-type oxygen analyzer. The results show that for a downward-moving packed bed, the direction of the void-gas flow in the downcomer to be stripped is important. For void gas initially traveling down (relative to the wall) in the stripping section, stripping-gas requirements could be decreased by: decreasing the solids flow rate, increasing the standpipe diameter, raising the stripping-gas injection point, or increasing the pressure drop in the stripping section. For gas initially traveling up the downcomer relative to the downcomer wall, stripping-gas requirements could be decreased by: increasing the solids flow rate, decreasing the standpipe diameter, lowering the stripping-gas injection point, or decreasing the pressure drop in the stripping section. A model was developed, based on the relative solids-gas velocity in the stripper, to predict stripping-gas requirements for nonporous particles. This model predicted stripping-gas requirements for the sand and siderite material to within 20%.