The objective of this study was to compare personal exposure to particulate matter (fine and ultrafine particles) in commuters using different transport modes (bicycle, bus, car and subway) in a busy, assigned route in downtown Santiago, Chile. Volunteers carrying personal samplers completed scheduled commutes during the morning rush hours, while central site measurements were conducted in parallel. A total of 137 valid commutes were assessed. The impact of central site, traffic and other variables was explored with regression models. PM2.5 personal concentrations were equal to or slightly above central site measurements, while UFP personal concentrations were above them. Regression models showed impacts of both background levels and traffic emissions on personal PM2.5 and UFP exposure. Traffic impacts varied with transport modes. Estimates of traffic impacts on personal PM2.5 exposure were 2.0, 13.0, 16.9 and 17.5 mu g m(-3), for car, bicycle, subway and bus, respectively; while for UFP exposure were 8400, 16 200, 25 600 and 30 100 counts per cm(3), for subway, car, bicycle and bus, respectively. After controlling the central site and transport mode, higher temperatures increased PM2.5 exposure and decreased UFP ones, while the wind direction affected UFP personal exposure. In conclusion, we found significant impacts of both central site background measurements and traffic emissions on personal exposure of volunteer commuters in an assigned route in Santiago, with impacts varying with transport modes.