The objective of this study was to improve the vehicular emissions inventory for the light- and heavy-duty fleet in the metropolitan area of São Paulo (MASP), Brazil. To that end, we measured vehicle emissions in road tunnels located in the MASP. On March 22-26, 2004 and May 04-07, 2004, respectively, CO, CO2, NOx, SO2, and volatile organic compounds (VOCs) emissions were measured in two tunnels: the Janio Quadros, which carries light-dutyvehicles; and the Maria Maluf, which carries light-duty vehicles and heavy-duty diesel trucks. Pollutant concentrations were measured inside the tunnels, and background pollutant concentrations were measured outside of the tunnels. The mean CO and NOx emission factors (in g km(-1)) were, respectively, 14.6 +/- 2.3 and 1.6 +/- 0.3 for light-duty vehicles, compared with 20.6 +/- 4.7 and 22.3 +/- 9.8 for heavy-duty vehicles. The total VOCs emission factor for the Maria Maluf tunnel was 1.4 +/- 1.3 g km(-1). The main VOCs classes identified were aromatic, alkane, and aldehyde compounds. For the heavy-duty fleet, NOx emission factors were approximately 14 times higher than those found for the light-duty fleet. This was attributed to the high levels of NOx emissions from diesel vehicles.