The isoprene atmospheric oxidation products methyl vinyl ketone (MVK) and methacrolein (MACR), along with CO, were measured in Toronto, Ontario in the winter and summer of 1996. These carbonyl compounds were highly correlated with CO in the winter, indicating that they are produced in significant amounts by automotive sources. Regression of the observed MVK and MACR concentrations against CO leads to emission factors relative to CO of 1.4(¿0.3)¿10-4 and 7.3(¿1.6)¿10-5 (mole/mole), respectively. Emission inventories for CO and isoprene allow us to estimate that, for Toronto in the summer, as much as 48(¿30)% of the MVK and 36(¿23)% of the MACR input into the atmosphere is derived from mobile source emissions. This source also has a strong influence on the MVK/MACR ratio, which in this environment remains relatively constant, with values typically ranging between 2.0--2.5. The impact of this is that MVK and MACR mixing ratios are not unambiguous indicators for isoprene chemistry and its impact on ozone production for urban environments.¿ 1997 American Geophysical Union