In this study the concentrations of the most important carbonyl compounds, formaldehyde and acetaldehyde, are measured in a highway environment in spring, summer and winter. Measurements were carried out in May, July and January along a two-lane highway, with daily traffic density 27 500 vehicles. Four sampling points were selected: (1) one from east and (2) west sides of the highway, (3) one at the central reservation of the highway and (4) one background site. The formaldehyde and acetaldehyde concentrations measured at both sides of a two-lane highway and at the central reservation suggest that the motor vehicle emissions are the source of these compounds throughout the year. The concentrations were, however, significantly lower at the roadsides than at the central reservation, especially in July. This could indicate rapid dilution of these compounds in the air. It is also possible that the vegetation along the roadside in summer may play a role as a sink or as a biogenic source for these compounds. The concentrations measured along the roadside in this study are comparable to those measured in polluted urban areas. The diurnal variation showed significant decrease of formaldehyde and acetaldehyde concentrations in the night indicating a decrease in primary source, i.e. traffic emissions. The seasonal variation was great as well, concentrations being lower in winter time, indicating lower secondary production of aldehydes. The formaldehyde and acetaldehyde effects on plants remains to be studied, since the formaldehyde and acetaldehyde pose a noteworthy risk to the vegetation exposed.