The effect on the modeled chemical climatology of the upper troposphere and lower stratosphere of including a limited set of nonmethane hydrocarbons in a two-dimensional (2-D) zonal average model is presented. Recent measurements of nitrogenated and oxygenated hydrocarbons in the upper troposphere and lower stratosphere have revealed the possibility of significant perturbation of this region. A zonally averaged 2-D chemical transport model enhanced to represent tropospheric processes was used to explore the extent of this perturbation on global and regional spatial scales and on seasonal and annual average timescales. Acetone was shown to cause a significant increase in the HOx budgets of the upper troposphere in the midlatitude Northern Hemisphere during the winter and early spring months, with acetone photolysis providing the most significant source of HOx radicals. The tropical upper troposphere has a uniform increase in HOx of up to 20% throughout the year because of acetone photolysis. Including the hydrocarbons caused a net increase in ozone of 5 ppbv in the lower and middle troposphere and 5--10 ppbv in the upper troposphere for global and annual averages. The effect of including the hydrocarbons on the calculated model ozone response for the case of doubled surface mixing ratios of atmospheric CH4 is also discussed. It is shown that including hydrocarbons in the model has a significant effect on the modeled ozone response to the methane increase. ¿ 1999 American Geophysical Union