We present examples of how chemical evolution can exhibit sensitivity to mixing arising from stratosphere-troposphere exchange. A chemical transport model is used to survey the chemical contrasts on isentropic surfaces that intersect the tropopause. Significant cross-tropopause gradients in both ozone and water vapor are shown to exist between 300 and 340 K. Back trajectories are used to confirm that air parcels with widely varying chemical properties are rapidly brought together in a typical quasi-isentropic stratosphere-troposphere exchange event. A two-box model is used to investigate the chemical evolution of stratospheric and tropospheric air parcels and to determine the effect of mixing between them. Mixing of stratospheric ozone and tropospheric water vapor is shown to lead to enhanced hydroxyl (OH) radical concentrations compared with background tropospheric and stratospheric values. The oxidation of CO, methane, and higher hydrocarbons is correspondingly increased, and NOx is also lost more rapidly with faster mixing. Also, in low NOx conditions, the rate of O3 loss is found to increase with faster mixing. The consequences of this anomalous chemistry for chemical transport in stratosphere-troposphere exchange events are discussed. It is also noted that if NOx levels in either parcel are very high initially, mixing of NOx can become more important than O3 or H2O in determining OH levels. ¿ 2001 American Geophysical Union