The chemistry of light nonmethane hydrocarbons (NMHCs), including C2H6, C3H8, n-C4H10, C2H4, and C3H6, was incorporated into a seasonally varying, one-dimensional model of the mid-latitude troposphere (45 ¿N). Simulations were performed for both marine and continental atmospheres. Where possible, these simulations have been compared with surface and aircraft measurements of test molecules, such as C2H6, C3H8, CO, peroxyacetyl nitrate (PAN), and peroxypropionyl nitrate (PPN). Model calculations predict that NMHC oxidation produces acetaldehyde and acetone in concentrations that can exceed that of formaldehyde. Organic peroxides were found to be present in concentrations that, considered collectively, are comparable to that of hydrogen peroxide. Peroxyacetyl nitrate is the dominant odd nitrogen (NOx) compound in wintertime and is surpassed only by HNO3 during the summer. The inclusion of NMHC-PAN chemistry has little effect on summertime odd hydrogen/odd nitrogen photochemistry, but is shown to have a substantial impact in wintertime, particularly over the continents, where NOx concentrations are high and temperatures are low. Under these circumstances, oxidation of NMHC compounds can enhance ground-level OH and HO2 concentrations by factors of 5--50, respectively. Formation of PAN during the wintertime can reduce upper tropospheric NOx concentrations by a factor of 2--3 over the oceans and a factor of 20--30 over the continents. Significant changes also occur in the predicted concentrations of HNO3 and HO2NO2. The effects of NMHCs on wintertime photochemistry might be moderated by seasonal variations in NMHC source strengths or by horizontal transport of trace gases from the continents to the oceans and from high latitudes toward the equator. Nonetheless, it seems likely that models of the troposphere must include NMHC-PAN chemistry in order to correctly predict the seasonal behavior of trace chemicals.