The aim of the present study was to identify the main sources contributing to the air pollution of three indoor environments of different use: a museum, a printery industry and an office. For that purpose, particulate matter (TSP, PM10. PM2.5), inorganic pollutants (NOx, SO2, O-3) and organic compounds (BTX, formaldehyde) were monitored. Factors such as the kind of the activities occurred indoors, the emissions from the existing equipment, the number of occupants, the ventilation pattern and the outdoor background substantially varied among the three sites. The average values of PM2.5 (151 mu g m(-3)), benzene (69.4 mu g m(-3)), toluene (147 mu g m(-3)), SO2 (47 mu g m(-3)) and NO2 (96.6 mu g m(-3)) were the highest values recorded during the experimental campaigns while all of them measured in the printery industry. Formaldehyde presented the highest concentration value in the museum (50.5 mu g m(-3)). O-3 was measured to have the highest concentration in the non smokers' office (238 mu g m(-3)) while the lowest was found in the presser section of the printer industry (11.0 mu g m(-3)). The location of the sites seems to play a significant role, too. Benzene/toluene ratio indicated traffic as a major source. Furthermore, ozone presented low levels in the printery and the museum (urban area) and relatively higher levels in offices (suburban area), indicating its outdoor origin.