This investigation quantifies the change in mass transfer within a confined gas volume subjected to mixing by loose spheres. A cylindrical vessel containing between 1 and 50 Teflon spheres in a tracer gas is vigorously shaken. Extractive sampling provides time histories of tracer gas concentrations extracted from the vessel. Fitting the results from a simple 1-D mass transfer model to the experimental data yields an effective mass transfer coefficient k ' for each experimental condition. Compared to diffusive mass transfer where k ' = D-ab = 7.58 x 10(-6) m(2)/s, k ' exhibits a cubic dependency on the number of spheres with a maximum at 17 spheres where k '= 3.5 x 10(-3) m(2)/s. (c) 2006 Elsevier Ltd. All rights reserved.