The impact of acetone (or any HOx source) on tropospheric photochemistry is largest in the high NOx regime. The fractional increases in OH and ozone production associated with acetone increase rapidly with NOx when NOx mixing ratios become larger than 300 parts per trillion by volume (pptv). This occurs in part because the HOx yield of acetone is larger at higher NOx mixing ratios, going from about 1HOx at NOx~10 pptv to 3HOx at NOx~1000 pptv. We also investigate the effect of acetone on the partitioning of the NOy family. Acetone increases the conversion of NO to NO2, HNO4, HNO3, and peroxyacetylnitrate (PAN). Conversion of NO to PAN dominates at low NOx, while conversion of NO to HNO3 dominates at high NOx. These NO decreases significantly diminish the increases in OH and ozone production one would otherwise anticipate from the increases in HO2. In particular, acetone can be expected to reduce ozone production for NOx<25 pptv. We also show that most of the changes in species concentrations arising from the introduction of a HOx source can be accurately predicted using simple expressions derived from linear perturbation theory. ¿ 2000 American Geophysical Union