Upper Atmosphere Research Satellite tropical wind and constituent measurements are used to estimate an upper limit of mid-latitude to tropical trace gas exchange by examining the phase of the N2O/CH4 ratio with respect to the Quasi-Biennial Oscillation winds. Assuming a simple diffusive model for tropical-mid-latitude exchange in the 20--28 km region, the mixing times are estimated to be at least 18 months, and, we estimate an eddy mixing coefficient of 7¿108 cm2/sec. This low mixing rate suggests that if significant chemical mixing into the lower tropical stratosphere occurs, as has been suggested by observational data, then this mixing must take place below 20 km.¿ 1997 American Geophysical Union