Five buildings located near roadsides (an office and a classroom with mechanical ventilation (MV) and three residences with natural ventilation (NV)) were selected with a view to characterising indoor and outdoor concentrations of organic (OC) and elemental carbon (EC). PM2.5 samples were analysed using a thermal optical reflectance (TOR) method for OC and EC concentrations. The average 24-h PM2.5 outdoor concentration was 78.4 mug m(-3), whilst the average outdoor OC and EC concentrations were 12.6 and 6.4 mum(-3), respectively, accounting for 17% and 9%, respectively of the outdoor PM2.5 mass. The average 24-h PM2.5 indoor concentration was 55.4 mug m(-3) of which indoor OC and EC were 11.3 and 4.8 mug m(-3), respectively, accounted for 22% and 9%, respectively. The mean value of indoor to outdoor ratios (I/O ratios) of PM2.5 was 0.80, with a close correlation between indoor and outdoor PM2.5 concentrations, especially in the NV residences. The average I/O ratios of OC and EC were 1.02 and 0.80, respectively. The higher ratio for OC reflects indoor sources of OC, which do not appear to occur for EC. The major source of indoor EC, OC and PM2.5, however, appears to be penetration of outdoor air, with a much greater attenuation in the MV buildings studied. (C) 2004 Elsevier Ltd. All rights reserved.