A one-dimensional canopy model was developed to study photochemical processes inside and above a mixed deciduous forest in southern Ontario. The Eulerian model made use of Lagrangian dispersion principles with a correction factor to incorporate the average ensemble time since emission to calculate atmospheric mixing; traditional diffusion methods were found to provide insufficient mixing to match the measurements. Neglecting chemical losses while making isoprene emission estimates was found to underestimate emission rates up to 40%. The ozone oxidation of biogenic and anthropogenic alkenes was found to be a potential source of hydroxyl (OH) and hydroperoxy (HO2) radicals during the morning and night. Reactions of HO2 with organic peroxy radicals formed by the OH oxidation of isoprene during the day were shown to be a significant source of organic peroxide formation above the canopy. The primary pathways of methacrolein and methylvinylketone formation were shown to be ozone oxidation of isoprene and hydroxyl radical oxidation of isoprene, respectively. Local ozone formation was shown to be limited by low mixing ratios of nitrogen oxides, despite high levels of isoprene present at the site. ¿ 1999 American Geophysical Union