The usual assumption by which chemical reaction rates are calculated in two-dimensional atmospheric models is by using a product of zonal means of rate coefficients and constituent concentrations rather than the rigorous zonal mean of the corresponding products. This assumption has been tested for the reactions O+NO2 → NO+O2 and NO+O3 → NO2+O2 using mapped limb infrared monitor of the stratosphere data from the Nimbus 7 satellite and found to be quite satisfactory for winter 1979 at 60¿ N in the upper stratosphere. Relative differences between the two-dimensional averaged rate and the more rigorous rate, calculated from the full, longitudinally varying temperatures and mixing ratios, were small (usually below 5%) and exceeded 15% only during times of strong dynamical activity. At those times or locations where stratospheric circulation is primarily zonal, the two averages agreed to within a few percent.