This study evaluated in-vehicle exposure to formaldehyde and acetaldehyde on actual commuting routes, while focusing on three factors (transportation mode, passenger car type, and commuting season). A total of 40 passenger car commuters and 20 public bus commuters were recruited. The same commuters participated in both the summer and the winter studies. The transportation mode and passenger car type were found to have little effect on the in-vehicle aldehyde levels. Conversely, the commuting season did influence the in-vehicle aldehyde levels. Meanwhile, the mean formaldehyde-to-acetaldehyde concentration ratios were similar in both the passenger cars and the public buses, plus there were significant correlations (P<0.0001) between formaldehyde and acetaldehyde concentrations for both the passenger cars and the public buses. This study also confirmed that, under Korean commuting conditions, vehicle interiors are an important microenvironment for exposure to formaldehyde and acetaldehyde. The mean in-car concentrations were 20.0 and 8.9 ppb for formaldehyde and acetaldehyde, respectively. Similarly, the mean in-bus concentrations were 21.2 and 9.1 ppb for formaldehyde and acetaldehyde, respectively. Furthermore, the in-vehicle formaldehyde levels were higher than those of a previous California study.